When buying S/H low hours is fine, but hour meters can be stopped/bypassed/replaced, so more importantly judge hours relative to the overall tractor condition - &, even if you know, ask the owner to show you where to check all the filters, fluids/lubricant points (including all the grease zerk, they should signs of regular greasing), it's a sure thing if he doesn't know the tractor has had scant maintenance..... similarly a fresh paint job can hide a multitude of sins.......& excessive panel damage is usually a sign the tractor has been abused.
Tractors if properly maintained/serviced (look for records/receipts of servicing/purchasing of maintenance components ) & used within their size capabilities should be ok for 4000-5000hrs easily (& many more hours) without major rebuilds - I've had sub25hp's acheive 6500+ & still going strong, I've also bought s/h "good" 150hp+ tractors with over 7500hrs, conversely I've seen one owner "new" tractors trashed by "weekend" amateurs at less than 1500hrs...
Apart from the normal mechanical checks, without knowing the specifc model or size range of the tractors you're considering, generally:
- the "weak" structural points which may highlight heavy abuse is around the gearbox/bell housing & linkage/PTO- look for signs of repair, weeping, metal fatigue/cracks/replaced bolts, front axles - impact damage? & steering components - again impact damage/fatigue/repairs ? & the 3PL lower arm pivot rod (close the tractor & where the lower 3PL arms connect) - quite often they're bent. The lower 3PL arms should not look abused/neglected, including the ball ends.
- check the fuel filter/bowl is clean (otherwise it's a sure sign of neglect), the air cleaner is free of dust (again of sign of signifcant neglect/potentail dusting & catastrophic failure), & signs of correct coolant in the radiator
- operate the tractor at working temperatures under load & check for overheating & smooth operation of all functions particularly PTO & smooth hydraulic functions: if possible lug the engine down under load to determine power, recovery & any strange engine/gearbox noises.
- check for signs if it's been used with a backhoe attachment, if so walk away (99 times out of 100, a 3PL backhoe in unskilled/semi-skilled hands will have done structural/metal fatigue damage to the tractor)
- the clutch, brakes & throttles should work smoothly & without hesitation, excessive exhaust smoke not under load can indicate potential injector/pump concerns.
- look for indications of recent dismantling/major work/components replacement: like new/replaced/missing bolts, areas where the tractor/component looks "mismatched" by being too clean & witness mark of fluid leakage on the machine or where it's regularly parked
- as new oil/fluid/coolant may also indicate the machine has something to hide....
Whilst tractor operating can be fun, it can also turn into a tedious chore if you buy a "dog" that requires costly ongoing repairs & remember it is a tool of trade not a toy so don't buy on emotions........ there'll always be another one if you're patient.