I would say, buy a model where there`s a reputable dealer nearby for parts and service, when in doubt probably get a John Deere. some breeds of tractors not everyone can work on, a shade tree mechanic might not be familiar and would take longer to learn how to fix $$$. Some brands don`t have parts available, I`d say if you find a tractor you like, visit the dealer and ask about parts availabilty and maybe the shop guy could tell you problems that model might have..
If you look for oil leaks where it`s sitting, if it has front weights it probably was used to do heavy work, look at the drawbar if it`s new or egg shaped which means it had a hard life. If it was repainted, probably hiding something. Make sure hydraulics and pto work, go though all the gears, make sure oils aren`t milky. When I buy a tractor and we come to a price, I usually say "i`ll drive it to my place and if it isn`t smoking, overheating or knocking...I`ll take it for the agreed price" usually if it`s a good tractor that is no problem for them.
I`d say under 6,000 hrs has alot of life left.
Good Luck
