The total tractor hours are not important. What is important is the component hours. You can buy a tractor with 80,000 hrs on it, and if it has been fully overhauled with all new components, it will perform as good as a new one.
This is what you get with a Cat Certified Rebuild. Cat sales brochures from the 1940's show quite a number of the old Cats (now, "antique" Cats) with huge hours on them. One sugar cane farm in Hawaii had 5 gas Caterpillar Sixtys in the 1940's that had all done between 80,000 and 100,000 hrs.
Cat say that most components are good for 10,000 hrs under average operating conditions. That drops to 7500 hrs under severe conditions, and can go out to 17,000-18,000 hrs under moderate working conditions, with good care.
If you buy a tractor with 7500 hrs on it, then on average, 3/4 of its working life is behind it. If it has been lightly worked, stored under cover, and maintained without regard to cost, then you are probably buying a tractor that is halfway through its component life. The problem is that only a small number of owners maintain their equipment properly, without regard to cost.
Most used tractors suffer from delayed or ignored maintenance, and are usually poor buys. If you can acquire the repair history of a tractor, you're way in front.
I'd rather buy a tractor that comes with a long list of recent repair expenditure, than one with no repair expenditure history at all.