This goes for buying ANY car:
1)Check the maintanence records,you want to know how well the car has been taken care of/or not taken care of
2)Check body panel spacing making sure all the spacing between the fenders & doors are the same,etc.If they aren't,ask the owner if the vehicle has ever had any damage resulting from a accident.Make sure and look down the side of the car for any noticable imperfections in the body itself.
3)Test drive the car and see how it behaves in town AND out on the highway paying attention to how well the car steers and listening for any odd noises(rear-end howling,rattling noises,etc)
4)Check underneath the car for any visible leakage from the transmission,rear-end.You might to on this step,take the car to a place(mechanic's garage) where you can get the car up onto a lift so you can actually see how everything looks.
These are some things to check,I'm sure others will give you some more ideas
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Paxton Blown 87GT