mine was priced similar new, as were many others with a few bits, plenty were a fair amount more,
it wouldn't be worth 12k imho
if it were a car with all the JCW options and a JCW210 perhaps, so with Chrono kit, Visibility, storage, Recaros, Carbon dash and exterior or interior bits with all main dealer history as and when required or something (but it won't have) and all 100% original BMW parts etc, and original paint, then it would be worth a premium, all in the eye of the beholder etc, currently you're facing paying over GP money for something that will take another 20 years to become an investment assuming you also don't use it, so it'll be a slow and small return, remembering these are not a low production vehicle and carry mileage quite well so, so what you see or the seller sees in it, many others won't I'm sure. You can find a nice car for 4-6k and make it quite quick for a couple more, discreetly tuned, looking totally standard, and it could possibly look just as smart cosmetically, with you just as if not more pleased with it.
unless you're going to show it as concourse in Mini events otherwise, I'd suggest keeping your money, that's if it really is concourse, rather unlikely in truth, 20k and 'clean' maybe, but as clean underneath as everywhere else? I doubt it.