Oh man, what to look for will be a long list, especially if you don't really know a lot about boats or the mechanical bits.
Checking the number of hours would be a start, the fewer the better, but really won't give you a clear indication of the boat's condition. You're best bet, hands down, if you don't know anything about boats, is to hire a certified marine surveyor to go over the boat top to bottom. They normally charge X dollars per foot. You may think it is costly but look at it like this, he may save you a ton of money by preventing you from buying a boat riddled with problems.