Your mechanic should give it a compression test, look for AD's, cut the
oil filter, sample the oil, look for corrosion. You should be able to
check the paint, upholstry, cleanliness, avionics and flight
characteristics.

As for old engines, even with low engine time, don't expect an old
engine (over 12 years) to make it to TBO. Frequently flown is MUCH
better. Frequently flown engines make it PAST TBO (frequent being over
100 hours a year, spread out through out the year).