Look to see that it has the aluminum crossover on the intake manifold if it is a 99. The plastic ones were prone to leak. Drive it and make sure it shifts OK in each gear. The synchros/bearings on the higher mileage cars tends to wear out. Also listen for a high pitched squeal that goes away when depressing the clutch. That is a sign of a bad throw out bearing. I wouldn't pay over $8k for a lower mileage one.
Also, try to look for one that is mostly if not completely stock so there's a better chance that it wasn't beat on. Some people will try to revert their cars back to stock when selling them, but some things are just too much work to try and switch back. $7 or 8k for about 80k miles, $5 or 6k for 100k miles, those numbers will apply more to the 02 03 04 gt anything older will cost less.