Looking underneath, as you said you will do, is at the top of the list. Interior items to look for are rust underneath the carpet. Actually pull the carpet back and look underneath to see if the interior has been flooded before. This can easily happen if caught in a rainstorm with the top off or down. Make sure every switch works. Check the horn; that may sound silly, but I bought a 1997 assuming everything worked and evidently the clock spring in the steerings wheel got wet and the horns did not work at all. I'm skeptical now if my airbag would have worked if it needed to. Just something to check. Another thing that goes out pretty often on TJs is the fan speed switch. I happen to have a 2001 that all 4 fan speeds still work, but when I was looking I came across several Jeeps of varying age that only one or two fan speeds were working.