-
There don't seem to many at the moment, but when I was car hunting before e46 convertibles were always surprisingly cheap (£2-4k). So as I said thinking out the box an M3 or 330i would be a cheap rear wheel drive option.
All that said a quick look for track and ralley cars looks like they sell for about 5% of their build cost. So I'd just buy someone else's toy then swap and sell the most expensive bits.
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F264041642595
Was going to suggest a left field suggestion of an e46 m3 convertible - as they are often supper cheap, then came across this for less than your next service.