Rear arches are about £50 each tops. I had a pair fitted to my old S2 a few yrs back. Will take that rust out easily. Get a good bodyshop to do it and it will look original. Couldn't even tell mine had been done.

Don't scrap it and re-shell into non-RS. If they are the only probs with the shell, then you have a good one. I personally have nothing against the reshelled RS's (like doo's old one which was/is mint after he did it..and was on ebay last week !) but for what you have: bit of restoration is all it needs. Besides, you can't then genuinely call it an RST..only an RST replica/reshell: no matter how mint it is.