You should budget for a few thousand to get it back to ORIGINAL condition, and that includes the wheels.
I can't stand seeing a classic that's had all sorts of non standard parts fitted,and non period mods done, but it's all about personal taste I suppose?
3/4k should have that looking mint I reckon, but as you say, the rust needs to be thouroughly asessed.
Subframe is the BIG one.
I reckon it could be worth a tidy sum if renovated to a high standard.
Original mint mk1 Cooper S' s are worth 25-35k now apparently!
Yours wouldn't fetch that obviously, but 7-9k should be attainable if you wanted to seel it I'd have thought.
The Mini owners club would give a better judgement on that though.
Good luck with the viewing and I hope it all goes well.