Rust is going to be your main concern when looking for a Pre-95 Wrangler. Check the undercariage very carefully. If its covered in extreme amount of undercoating and spoungy to the touch they are probably hiding massive rust.
Pay special attention to the driver and passenger floor panes, and the frame, especially from the rear shock towers back.
Here is example of how mine started to rust even though i had a undercoat.
Use your fingers/hand and push up on the floor pans.
A big thing I do is look and feel the car, take your hand and lightly glide it over the surface area's. Your looking and feeling for waves, high, or low spots. These can tell you if its been wrecked, or damaged in someway.
Cosmetics asside if the engine and trans are rebuilt then find out by who, did they give a warranty with it. ( In my experience most trans rebuild shops give a warranty) [At least around here] 1 year]
Finally even though the Jeep is beautiful in the pictures, I wouldn't pay more than 5000 to 6000 for it. If you get there and its been pro rebuilt trans/engine and is imaculate condition I might go 6500 to 7000 for it.