Minimum wall clearance? How much of the framework should I retain - or should I buy new?

If you're talking about access space between the wall and layout 18" would be the minimum IMHO.  If you talking about visitor space to view the layout 36" would be the minimum.  Again this is just MHO.

Really depends on how much you can squeeze.  I have 18" walkway on one edge, but I don't plan on spending much time there.  Best thing to do is simulate the walkway with boxes and see what spacing you are comfortable moving in.

I have about a foot on the back side of my layout. I only go back there for derailments/problems though. It is pretty tight for me. I can turn around, but I have to squeeze in a bit do so. My grandson has no problem flying around back there, but he is only 12 and a bit smaller in height and width than I am. I would recommend the 18" minimum, as others have stated above, if you can get it. Of course that would be just for access and you probably wouldn't want to stay there too long. More is better! I also agree with the 36" mentioned above for viewing access for visitors.