I think he summed it up pretty good. These cars don't usually have any major problems, but the transmission syncros can wear out, and the shift forks can bend with a heavy handed shifter. Both of which will usually make a grinding noise, and can cause the tranny to pop out of gear. The best thing to do is drive the car at all different speeds, and try accelerating pretty hard through the gears to make sure it runs right.
Other than that just open the hood and look everything over. If you can try taking a quick look under the car to look for major oil, transmission fluid, and coolant leaks. As was posted above the rear main seal and oil pan gaskets can go bad on these cars and they're a PITA to change so just keep that in mind.
Also when negotiating the price just assume it needs new brakes, oil change, etc because they usually do.