Good you've went and seen it. I had my doubts about the bottom sill areas from the pictures. The sills can cost £2k per side to put right although I got mine done much cheaper than that with new panels.
I had a project S2 cab a few years ago that needed a new rear screen. That can be costly too but I think I got it done to an acceptable standard for about £400 IIRC from a local upholsterer. Leaks from the soft top are another concern and new seals are an absolute fortune.
However if they sort out the tin worm, rear screen, no leaks from soft top and engine is in good nick with recent SH then £20k is a steal!
Stuart