From what I've been told, the average person drives about 10,000 miles a year. So you could go by that. So if someone drove it every year up until now you'd be looking at around 150,000-170,000 miles. I'd consider low mileage to me anything under 100k, but that's my own opinion.
Personally, I've never owned a supra, but have been looking for one for awhile now (my budget and the prices disagree). When I looked at an '87 supra a few days ago, the first thing I checked was for head gasket problems. I looked in the radiator and founds globs of oil (that's not supposed to be there
) so I'm kind of worried about buying it. Another problem I've heard about is the hatches leaking water. So I guess you should check around in the back for wet/moldy carpet?
Turbo vs N/a? It's probably just personal preference here. If he just needs the car to get around, I'd lean towards the non-turbo model. The turbo model has an extra 32 horsepower (and is capable of much more) but the non-turbo comes with a nice 200 horsies.
As far as finding them goes, good luck. There are only about 5-10 for sale within 50 miles of me and most seem to be damaged in some way or another. Good luck with finding one.