Being it is currently in use is a good sign.
They were good reliable tractors for their time.
Check the engine oil for coolant, look under the oil cap and at the end of the vent tube. Some condensation is OK, especially if there is no thermostat. But if there is anti freeze, could be a cracked block, had gasket, or cylinder liner orings leaking.
If it lives outside, expect to find water in the gear cases. Good news they are tough, it usually doesn't hurt them, will just need changing.
The 3 point should lift smoothly without jumping. Jumping while going up is a sign of pump problems.
If you can run it long enough to get the engine up to temperature, see if it has oil pressure at idle. It won't need much, but should see something on the gauge. If the engine will idle slow and smooth, the compression is good.
If it's been used for loader work, expect front suspension and steering gear problems. Look at the bolts that hold the front end to the engine, be sure nothing is broken or been run loose.
On something that old, expect the typical, bad wiring, bad radiator, bad gas tank. Unless these things have already been addressed, good chance they will need replacing sooner than later.
But keep an open mind, they are still good tractors, you are just looking at something getting up in years, expect some age related issues. Remember, it does not have live PTO. It will need an overrun clutch for mower use.