That shell does look good tbh, especially for the price (apart from the all-too-common bent floor and rail - idiot's jacking them on the floor pans...). TBH, you could make it work, given how much aftermarket kit you've bought, the engine swap, etc. It's just certain items wouldn't work, e.g. doors due to the latches, etc. Late 280Z fuel tank wouldn't fit so you'd have to get an early one or go custom (both can be had new). It might require some experimentation as you go along, but nothing insurmountable. Not compared to what you were facing with the fixing the grey car! Most of the important "big" stuff will just bolt in. Given you are planning a RHD conversion too, and will probably end up redoing the wiring extensively to integrate the RB, etc., the electrical side of things could again be made to work in that shell.
If its still available, have a crack at it if that standard 280Z you've been offered doesn't work out.
Old shell: I'd give it the angle grinder treatment personally. A handful of sections could have some spares value. The downside of selling it complete is that it raises the spectre of some arse building it into a complete car without genuinely fixing it, creating a "rinse and repeat" situation to what you've just had.