Well, its an older car, so deffo go and knock around the car for rust issues, look from below, if there are any oil leakage, mainly in transmission and under turbo, see how is clutch responding, try to shift between gears a bit faster. see tyres of uneven tyre wear. Dont let him start the car before you are there, if he does, wait for it till it cools down completely and start it from cold, give a listen for unusual knocks.
!!!When turning on the key, pay attention that ALL lights on dash come on, including check engine light light!!!
ask loads of questions, when was oil changed, what oil was he using, when was timing belt done, etc etc etc etc...
And remember, even with the best inspection of the car, it might look and sound super good, after a week of driving, something will appear that you did not notice before...