I would look for easy-starting cold and good oil pressure first off. Next, check rear tire condition and sheetmetal as those are expensive parts to replace. Make sure the clutch works well if standard, and the tractor goes ok in all gears. If it has a loader, check for broken welds, bends, and cracks, especially if 2wd as those are real troublesome with loaders compared to 4wd's. Look for oil puddles or other signs of leaks under engine, rear end, hydraulic pumps and cylinders, etc. Pull the dipstick and note the color of the oil (milky is bad - means water is mixing in there). Check the steering for excessive free-play. If you are looking for gently used but older machines, I like to find the ones still wearing factory paint, and indication of a life "indoors" when not in use. The real old ones were mostly 6-volt electric, and I would steer clear of those that have been converted to 12 volts. You know you have a tight engine with lots of life left if it still starts good when cold on 6 volts, while a practically worn out motor will start ok cold on 12 volts.