Buying a boat could probably be the same as buying a house. List out what you "need" and "would like to have". Once you have this you should be able to start your search.
i.e.
Need:
Boat w/ working motor.
Trolling motor
Fish finder
Want:
Shiny fiberglass boat that will put me on the far side of the lake in 5 secs.
Trolling motor that is GPS capable/integrated with fish finder.
Like the others have said, its what your budget can handle; hidden costs, maintenance, gas, insurance, taxes, ugg.
If your looking for that awesome deal, well you are just going to have to keep your eyes and ears open, it may take a while to find.
With that said, I bought a new boat for the warranty and to make sure the motor wasn't going to die in a year, btw motors are expensive, normally more than the boat.