Buying a used tractor is just like buying a used anything else. Look, look, look, get a feel for the market and then make an informed decision.
My wife and I scoured central OK looking for the right tractor setup at the right price. The more you look the more you get a feel for what is a good deal for your particular market and what you should run from.
We picked up a 1980 Long 34 hp w/ FEL and Brushhog for 3k, not sure of the total hours, but since the meter was broken I got the guy to knock more money off of his asking price. It's run fine and I've had no complaints.
Another consideration is how long you plan to have it. Obviously you don't buy total junk, but little things that need to be fixed sure do work well to lower the asking price. Personally, I needed something that would last until I finish school (this December) and then a year or two after that. By then the tractor will have paid for itself in saved time and back aches.
Clint.
P.S. Billy, cool goatee, I've got a red one just like it.