18"s on any SLK of any year would look great... maybe if you are into a comfort ride and easy boulevard cruising, then you are better off on 16s and 17s... However, if you have any interest in your sport-lux R17x delivering some sort of sporty ride and handling, then those 18s would deliver this in spades with slightly wider rims and slightly lower profile tires.
There is so much made into concerns for wheel upsizing and some are just myths....
Why not go test drive a R170 on 18s and one on 17s to confirm if the warning about upsizing is real or imagined...
My R172 came with AMG 18s and I am riding on AMG 19s... MB builds these convertibles to handle up to 20s as you'll find in the AMG GT, SLR, SLS, etc... sure these are bigger cars with bigger motors, but our R17x SLKs can sure handle up to 19s with aplomb...