Good on ya for taking the gamble and buying it. I know a guy who has been looking for years and will probably still be looking because he is too picky. One of those dreamers who wants an A-class condition car but is not willing to pay the appropriate asking $$. Even with the amount of rust so far I think it's still a good buy Des.
When I was inspecting my Z before I bought it I took along with me a small magnet which I used to test whether an area had bog or not. It's a very simple and crude method but it works. You can quickly tell whether an area has no bog or heaps of bog based on the intensity of the adhesion. Same thing applies if an area is rusted away, it usually won't stick that well.