i think you would be putting on an un-necessarly small tyre, best to stick to 225/40 and get the correct offset, at the end of the day if the wheel itself is to far out the tyre will still rub.
Where is the best (Cheapest) place to get the BBS CK or CH's from?
Stan would I get away with 8J x 18's with 215/35/18's ET 38-42