Your top brands are John Deere, International, Ford and Allis-Chalmers (made until mid 80's).
Basically, nothing runs like a Deere except for you when you see the price tag so expect to pay high prices for the top tractors as farmers know what they're worth and are usually willing to pay for it.
A well taken care of tractor will run forever. We had an old International with almost 70,000 hours before we sold it. You want to make sure theres no transmission damage/major repairs done (indicates something else might go next) and there is no major stress marks on on any of the metal. Other than that, just check over all the normal stuff you would when checking out a vehicle.
I'm not sure what he wants it for, but just a warning, they can be pretty dangerous. I have 15 years experience on them and I've almost killed myself numerous times. They're not toys and if you treat them as such, they can kill you as they can tip quite easily.