Well I'd say engine condition is a top priority. If it smokes much, that can get to be an expensive fix.
Get it running, put the deck in gear at low throttle. Open the throttle all the way. The engine should come up to full
speed in a second or two.
Get it running along the ground at a slow mowing speed, with the mower deck engaged and with the throttle all the way open. Then push the ground speed lever all the way to fast. The mower should accelerate rapidly to full ground speed without hesitating or lugging the engine down.
Does it start up easily? Cold? Hot?
Next would be the condition of the hydrostatic transmission. The machine shouldn't have to "warm up" more than 3 or 4 seconds before the transmission will work. Also, the machines shouldn't gain much speed going down a hill or loose much speed going up a hill.
Does it come with a mower deck? If so, are the bearings noisy? Or loose? They shouldn't be either.. Is the deck sheet
metal in good condition? Some have rusted away and had patches welded on. That's not all bad as long as the repairs
haven't caused any alignment problems. How much life is left in the mower deck belts?
Does the pto clutch engage and disengage properly? The clutch must not slip once it's engaged! If you mow into some reallyheavy grass, the engine should bog down some. If it doesn't then the clutch is probably slipping.
I think I'd want to try to mow with it for an hour or so to see what it's really like.
Of course there's the steering---- How much slop is there in the whole system? Can you drive down the driveway at full speed without having control problems?
Does the gas tank leak?
If it has lights, do they work?
This model should have an ammeter. Does the starter/generator actually charge the battery?
Sometimes it's hard to tell because the meter bounces around a lot, so you might take a multimeter with you. Check the battery voltage with the engine off, and then again when it's running.
The voltage should be about 12.4 to about 12.6 with engine off and at least a volt higher, to as high as maybe 14.6 volts when running.
Turn the lights on, if the tractor has them. The battery voltage (engine running at medium speed) should stay above 12.6.
It it has an electric lift, does it work properly?
If it has a hydraulic lift, does it work properly and how much life is left in the pump's drive belt?
Check condition of all belts. Generator, PTO to deck and deck to blades. They are pretty expensive these days. Note: I have never found a PTO to deck belt that works well other than the actual Cub Cadet belt!
Be on the lookout for minor missing parts that might be expensive to replace.
We have a 105 with a tiller. It definitely has enough power. Personally, I wouldn't even attempt to use a tiller on one without the hydro. They made some of the standard transmission models with a "creeper gear" that might work, but if you are in sod or in hard-going soil, you really need the option to go REAL slow and to micro-adjust the forward speed.
First gear in the standard transmission isn't anywhere close to slow enough.
Make sure you have a good, fresh pto clutch because the tiller puts quite a load on it. Lastly, the looooooong belt that transmits the power to the tiller mechanism from the pto shaft is VERY expensive, so take good care of it.