If you have a friend who is any sort of mechanic, have him inspect things on a car you might buy. Lacking that, here are some generic guidelines -
As far as mileage, that may or may not be an indicator of how much longer a car will last. If a car has over 200,000 miles, THEN I would say avoid it because even the best cars have a lot of wear by then.
Even so though, if a car has a lot of small problems or looks like it was abused, it is safe to assume it will give you a lot of headaches.
If it looks like it has been taken care of and doesn't seem beat to hell, it is likely okay.
Also, now this is not P.C. to say but here it goes - If you buy from a private owner, look at the person or people and ask, "Is this someone I feel comfortable doing business with?" If it is some guy who looks like he has been featured on Jerry Springer, you do not want the car. If they seem like decent people, probably okay to take a chance.
Some sellers say "It has this or that problem but it is an easy fix/cheap part at the junk yard". Walk away. If it really were an easy/cheap fix, they would have done it to make the car more marketable.