Buying a used car is, unfortunately, never more than a carefully calculated crapshoot. Hate to say it, but it is. Sometimes they work out wonderfully ,sometimes they don't, and often times there's just no way of predicting which is which.
We have been very fortunate in having purchased two used vehicles that have worked out fabulously for us. We purchased a used Mercury Villager in '01 and got seven years of terrific service from it. We picked up a one-owner '04 Ford Ranger with 40K original miles in absolutely mint condition for right at $2K below what most sources indicated we'd pay, so good deals are definitely out there to be had.
Like anything else, shortening the odds takes a bit of legwork. Pick up a copy of "Lemon Aid," a used-car guide that talks about which used cars to avoid like the plague, and which are better bets. Consumer Reports has a used car history for most vehicles, and can give you a good indication of which varieties are more reliable than others.
It is very difficult to guarantee that a used car isn't a flood damaged item, because floods happen everywhere and the "real" water damage can be very easy to conceal. For any prospective purchase, especially from a private party, insist on the opportunity to have the car evaluated and checked out by a mechanic. They can at least check out some conspicuous areas for rust that might be indicative of possible water-induced damage.
Used car warranties are very, very dicey propositions. Most use car dealers either don't offer them, or charge an exorbitant fee. And if they do offer them, they come with high deductibles and exclusions and are honored only at specific locations. I see an increasing market for third-party warranty companies that seem to be willing to warranty any car for a price, but I have no experience or information on their reliability. Bottom line - just be very, very careful.
The experience of buying new being almost as cheap as used bore out for us. I was looking for a used minivan in '08, and found the prices of most vans in decent condition to be within a stone's throw of full retail! I was flabbergasted. So we ended up going against instincts and buying a new Sienna, and we've loved it. So there's that consideration as well.
Buying new cars can, to a degree, fall back on a formula that helps you maximize the amount of car bought for the dollars spent. Buying used cars is much more black art than science, because there's a severe limit on how much you can know about any used car that someone else, for whatever reason, doesn't want anymore.