I would ask to have it put up on a lift so you can get a real good look at the undercarriage. If he wont agree to that, ask why.
As far as price, every car stands on its own. Engine size, trans, options, and of course condition, are what determines the cars value. You can have a wide range of prices for two exact model year Stangs depending on the above factors. I would look at ebay to see what that particular model is being bid at to get an idea of market conditions.
Looking at the underside of the car is a good recommendation but not good enough (poarticularly on a convertible). Floor pans rust easily and the rust from the inside out more often than from the outside in (again, particularly convertibles). Take a phillips screwdriver with you and pull one of the front kick panles and sill plates, then peel the front outside corner of the carpet back. Also remove the back seat (no tools required at all) and look under the backseat for rust, the top-well often leaks on verts and will rust the back seat pan. If you see ANY rust in either of these locations knock $2500 off the price. If the owner wont let you do this knock $5000 off the price assuming you don't walk away.
You can also use that phillips screwdriver to poke all four framerails, the metal should be VERY solid and undentable.
darn rusty underneath, interior = shot, and the drives side shock tower was a bit bent up from a front end collision. i think i can find a better car for the $$