Go to: http://www.z31.com/lookfor.shtml

This is a good guide on what to look for on a Z31 (84-89 300ZX).
Also check out the FAQ pages there - the best site for these cars!

These are great cars, but have a limited market. Don't expect to be
able to fix it up and sell it for a very high price. But that's ok, they
are so much fun to drive. These are "real" cars with rear wheel drive.

If you want to go fast - get a turbo model (you can also change a
NA to turbo fairly cheap, but requires work). Clean turbo models
take a little more time to find.

A common problem is vacuum leaks that can cause the AC problem
that you mention (could also be more to it).

Make a list of whats wrong with it and add up the cost to fix it up and
see if it makes sense to you.

Be patient when shopping for a Z. Good ones become available all the
time so don't settle for something that will be a major headache.

Good Luck,
steve