964 is an excellent, but underrated car, hence the cheap prices.
There are a list of common issues involving oil leaks, dual mass flywheels, distributor belts and suchlike, but if you go to rennlist.com there is a wealth of information on what to spot and what to buy.
Perhaps most importantly on a high miler (as the often are >80k) is that the biggest expense is a top end rebuild (circa £4k-7k) depending on who you go to and what you have done, but once completed you have the very great possibility of trouble free motoring well into the 100k mile territory.
I'd buy one, in fact I probbably will, if the right one comes up. They are faster than they should be for the power, reliable, have fantastic handling and control feedback, plus they are immeasurably cool cars.
The best bit? Pick one up for 15-18k and provided its "good" and the engine work has been done, you are very unlikely to lose money on it /ubbthreads/images/graemlins/smile.gif My friend has had one now for about 3 years. Cost him £15k and it's still worth that. His has some a very minor oil leak, suggesting that in the next 10k and engine rebuild is due, but if you had bought any other car 3 years ago, you'd have lost more than the £4k it's going to take to put the porsche right in depreciation - clever car, clever money /ubbthreads/images/graemlins/smile.gif
BTW, the 993s are overpriced still, in high demand, and don't offer an awful lot more... You can buy a 996 cheaper that a decent (and older) 993 /ubbthreads/images/graemlins/shocked.gif