I'm sort of knowledgeable in this area, as my company makes 3 to 5 lug Smart adapters..
Smartparts
I've spent a TON of time under Smarts measuring, and I'd recommend no bigger than a 17" wheel, for a couple of reasons:
- It's possible to get 17" tires with circumferences VERY close to the original 15" tires, ensuring that the speedo remains correct and that the stability and traction control don't get weird inputs from the wheel speed sensors.
- 17" rims come fairly close to the lower spring perch on the struts; 18's would likely either make contact or be really scary close.
- 17's really seem to fit the scale of the car without looking "cartooney".
- I suspect that you'd have some interference problems regardless of the front rim width or offset, just because of the increased overall diameter of an 18" tire.
We've had really good results with a 195/40/17 on a 7.5" rim, and up to a 225/35/17 on a 8.5" rim out back. Keeps the revolutions per mile the same as stock, fits up nicely, looks appropriate and frankly, the world is chock full of OEM 17" Benz take-offs.
Cheers-
Johnny