[84LX89GT]

This is what i look for when buying a mustang:
1. Oil leaks, check underneath at the rear of the oil pan, oil travels down and back....old fords love to leak oil. also for any other oil/fluid leaks
2. Make sure the heater works and blows hot, heater cores are not fun to replace. i just replaced the one in my car, not bad, but a pain in the arse anyways. While your at it make sure it blows cold air if it's got AC
3. Check THOROUGHLY for repainted areas or any part of the frame (even underneath the car) for crimps and crumples for signs of an accident, mustangs are never the same after they've been totalled/frame damaged because of the unibody.
4. Check how it runs cold and take a short trip cold, make sure it shifts ok and there's no odd noises. park it and let it run, then take a longer trip while it's nice and warm to make sure it doesn't over heat/miss or do anything funny and that it still shifts ok. Also make sure there's no clunks while applying power/releasing power this could be anything from worn trans splines, worn universal joints, excessive gear backlash, or ripped torque boxes (holds the fronts of the controls arms, notorious for ripping under abuse).
5. ask for any maintainance history the owner can give and check the dipstick for tarnishing (dark areas on dipstick). Vehicles with irregular/extended oil change periods will stain the dipstick and in some cases the dipstick will get rusted/eaten by the acids in the oil (happened to a family friends truck).
6. Inventory all the things that don't work/worry you. It can be used as leverage in negotiations. Also DON'T fall in love with the car when you first see it....i've burned myself before doing that.

This is at least what i can think of off the top of my head. Good luck and happy hunting.