First Principles
Finding an affordable boat that can take you anywhere is not easy, but it can be done. There are many different types of boats out there, though, so the first step is to know what you're looking for. Boat characteristics can be broken down into a few major categories: length, sail plan, hull type, keel type, and displacement.
Length
32 feet is enough. Forget about climbing up the rat lines to the crow's nest so you can gaze out past your three masts towards the horizon. Small boats are where it's at. We can examine this from two perspectives: cost and performance.
In terms of base cost, the total surface area of a boat is the square of its length -- so cost increases rapidly as boats get longer. The difference in cost between two comparable boats when one is even two feet shorter than the other is shocking. Longer boats also mean more of everything else: longer lines, bigger blocks, bigger sails, bigger anchor, more to paint, etc. This makes ongoing costs much harder to cope with.
In terms of performance, it is possible to sail a small boat anywhere you want to go with no engine. Big boats can offer a little more in the way of space and comfort, but it's not possible to sail in a crowded anchorage or short-tack up a narrow channel in them. This makes you dependant on your engine and on fuel. It's also a lot less fun.
Boats between 24 and 32 feet are more affordable, easier to sail, more versatile, cheaper to maintain, and more fun.
Hull Type
Boat hulls are typically either wood, fiberglass, or ferrocement.
While wooden-hulled boats make the heart go pitter-patter, they can also be a world of trouble. Once you have one, they need to be constantly kept clean and regularly repainted with anti-fouling paint. They will rot very easily and have more of a tendency to leak, but for these reasons used wooden boats are usually cheaper than fiberglass ones.
For a long time, ferrocement was thought to be the hull material for those of us who aren't yachties. Building ferrocement boats was much cheaper than the alternatives, but over the years the material has not served well. There are tons of stories about ferrocement boats hitting minor obstructions and the structural integrity of the hull disintigrating shortly afterwards. For this reason, almost no new boats are built with ferrocement, and only the boats that haven't sunk yet remain.
Fiberglass hulls are probably the most reliable, easiest to maintain, and most cost effective. Some fiberglass boats built in the early sixties can be particularly strong, since they were layed up like tanks before anyone realized the lasting strength of the substance. Boats built around the time of the oil embargo (1973-1974) can be of poorer quality, because fiberglass resin is a petrol-chemical that was being used in degraded forms at that time. In general, though, plastic boats are where it's at.
Keel Type
In general, the keel either runs the length of the boat (a full keel) or it is concentrated in one spot (a fin keel). Both have their advantages and disadvantages. The fin keel boat is more manouverable and generally faster, where the full keel boat is more stable. Most sailboats geared towards making long-distance voyages have full keels, but I personally like the manouverability and speed that the fin keel gives you.
With either type, the keel can either be bolted onto the hull or fiberglassed in. Keel bolts are a major point of failure on boats, and need to be checked closely.
Displacement
The amount of water that a boat displaces is just an indication of its weight. It depends on where the weight is, but generally the more a boat weighs the more stable it is. This does not mean that you should take a light boat and throw a bunch of weight in it, because light boats survive in rough conditions by getting tossed around instead of plowing through things. But a heavy keeled boat that's layed up with a very solid hull is going to be tougher and more stable on the open ocean than something lighter. What you sacrifice is speed and manouverability.
Sail Plan
The most popular sail plans available today are: sloop rigs, ketch rigs, cutter rigs, yawl rigs, and schooner rigs.
Sloop Rigs
This is the most popular sail plan available, as well as the easiest to sail. It consists of one mast and two sails: one attached to the mast and the other attached to the forward stay. This sail plan has good up-wind performance, and only requires you to adjust two sails. It also means that there are only two sails to get worn out or need replacing.
Ketch Rigs
The ketch rig is distinguished by the addition of a smaller mast behind the main-mast. It is thought that the advantage is in breaking up the sail area into multiple sails, so that each sail is easier to handle individually and there are more options available in terms of how much sail is up at any given time. It is rumored that ketch rigs do not point well to windward, however, and breaking the sail-plan up into smaller packages only makes sense on bigger boats anyway (where the mainsail becomes huge).
Cutter Rigs
The cutter rig is just a sloop rig with an additional forestay and jib known as a staysail. This is one of the most versatile sail rigs available, since you get a lot of the sloop rig simplicity along with the option of using a staysail. The staysail can come in handy in light winds when more sail area is needed, in extremely heavy winds when you take down the jib and only a small headsail is needed, or running straight down-wind when you sheet it in all the way to act as a roll-stabalizer. Also, staysails are often self-tending club-footed sails (mounted on a minerature boom), so if you have to short tack a long ways you can take down the jib and then tack repeatedly without having to re-sheet anything. The main disadvantage is that tacking with a jib up becomes more difficult, since the sail has to come back between the cutter stay and the fore stay. This is particulary difficult when using a very large jib.
Yawl Rigs
The yawl rig is a sloop rig with a very small mast on the aft of the boat. These are not commonly seen, but can have a few advantages over a sloop. Unlike the ketch or schooner rigs, the additional sail is not merely a means to break up large sail area. The small yawl sail can act as a good balancing sail, which can help with self-steering or keeping the boat pointed into the wind while dropping anchor. It can also act as a good storm sail in extremely windy conditions.
Schooner Rigs
Schooner rigs have more than one mast, where the aft mast is larger or the same size as the forward mast(s). These rigs have the potential of offering the most sail area possible on a given length of boat, since it is possible to fly topsails between the masts in addition to the sail area available. But mostly, the schooner rig is just another way to break up sail area so that individual sails are easier to handle on large boats.
The most comprehensive online listing of used boats is yachtworld.com. While this resource is vast, and there are some deals to be found, boats can not be sold on this website directly by an owner. The postings are all made by brokers, which generally means that the boats are going to be on the more expensive side. So the best deals (for small boats in particular) are usually found at the marinas -- especially the less-swanky ones. Look around for bulletin boards at the tops of the docks or in the harbormaster's office. Sometime reasonably priced boats also make it into the classified section of the magazine Latitude 38, and in the Bay Area there's always craigslist.org.
Another interesting possiblity is the Lien Sale. Most marinas have a problem with people abandoning their boats and not paying their slip fees. Once every few months, these marinas collect all their derelict vessels and sell them at silent auction. The minimum bid at a lien sale auction is usually around $200, although I'm not sure what the bids usually get up to. Before the lien sale, however, all of the boats to be sold are usually put on a "Lien Sale Dock" together. They usually sit there, unwatched, for several months. It does not seem beyond the realm of possibility to slip out with one of these boats in the night, and chances are that nobody would really care about coming after you.
It is worth recognizing that boats acquired from a lien sale dock -- like any beat up boat -- are not necessarily"cheap" or "free." Most are usually in pretty rough shape, and require a lot of repair. There are some clever ways to avoid ongoing costs, but in general marine grade parts are very expensive.
What To Look For
When checking out a used boat, be sure to inspect the following:
The Standing Rigging -- Try and discover when the standing rigging was last replaced. If it wasn't within the past seven years, chances are that you're going to have to do it soon. Run your hands up and down the length of the stays, feeling for "barbs" (protruding pieces of cable). If there are barbs on a stay, it probably needs to be replaced. Check into how much pieces of 316 cable at that thickness cost, and factor that into how much you're paying. Look at the turnbuckles (the bolts that connect the rigging to the chainplates) carefully -- check for rust and cracks/fissures.
The Keel Bolts -- Check out the keel bolts from the bilge. If they're rusted at that end, chances are that there's a significant amount of rust on the keel side. If they look really bad, think about what it would take to unbolt the keel and replace them.
The Hull -- If the boat is hauled out or if it is possible to haul the boat out, inspect the hull carefully. If it's fiberglass, check for "blisters." If it's wooden, look for any signs of rot or wood-worm. Either of these conditions are an ordeal to repair.
The Decks -- Most fiberglass boats have wooden-core decks (either plywood or balsawood). Often, fittings that are bolted into the decks are not mounted properly and leak slightly. This can cause the wooden core of the deck to develop dry-rot, which is a fungus that can spread to any wood that it is touching. Be sure to walk over every inch of the deck and check for soft spots, weak spots, or sagging. These can be an indication that the deck is rotting. Think about what it would take to replace those sections of the deck.
The Running Rigging -- Look at all the lines and check to see if the ropes are fraying or if the wire halyards are developing barbs. If they are not in good condition, think about what it would cost to find new lines.
The Sails -- Put the sails up and check for rips, tares, or areas that look threadbare. Are they crinkley or soft? If the sail covers are not in good shape, that could be an indication that the sails have not been well protected. Think about what sails you're going to need and what it would take to replace them if they're not functional.
If you're buying a cheap used boat, all of these things are certainly not going to be in perfect order. It is alright if the boat needs work, because that's why you're getting it so cheap. It is important, however, to keep in mind how much the work you plan on doing is going to cost. Even if you do all of the labor yourself, there are some additional costs that are mostly unavoidable. Woodwork, fiberglassing, aesthetic repair, and painting are not incredibly expensive, while repairs to the standing rigging, running rigging, or sails can be on the more expensive side. If you need to repair or repaint the hull, that means you're going to have to pay for a boat haul-out. Keep all of these things in mind while you're calculating what the boat is going to cost.
By Way Of A Conclusion
If you manage to find a boat and acquire it, do the minimum amount of work required to get it sailing and go. Set a date for departure, do as much work as you can before then, and leave on that date. Marinas and harbors are overflowing with sailors who are leaving "real soon now" or "as soon as they finish this next project." It is so common that sailors never end up leaving, and instead work on their boats into eternity, that everyone assumes those who say they're leaving "real soon now" never will. Don't fall into this trap. Go. Go now.