Will this work?

18"x9", 5x112, ET 31 will this fit on  the 08 S-line stock?

I hope so.
Thats what I want

I cancelled my bid on ebay because I read soemthing about the offset being too off. 

I would ask a local rims place if you know of any.  That way you get an answer from an expert since it seems to be a bit quite in this thread and I know you want a fast reply. 

Simple math: Stock offset is 56mm on a 7.5" rim.  A 31mm offset pulls the wheel out 25mm (1 inch).  Going to 9" wheels adds 1 1/2" in width, 3/4" inside and 3/4" outside.  So your net is 1/4" more space inside, but 1 3/4" less clearance outside - more than enough to rub on the fender opening. 

So it'll bolt up and work all the way to the 1st corner where you will rub and start cutting down the tire.

Rule of thumb:  for 7.5" Rims 45-56MM offset.  For 8" rims, 45 - 50MM offset. For 8.5" Rims 40-43MM offset. For 9" rims 38-40MM offset and rolled or flared fenders.  As you go wider you have to worry about both inside and outside clearances more, so the tolerances decrease. 

My recommendation is to stick with 8" wide rims with 45-50MM offsets and 225 or 235 section tires.  Anything beyond that will require lots of work to fit.

What rims are you looking to run?It sounds like AMG rims.

I was bidding on the Oettiner RZ wheels.  Good thing the seller agreed to cancel my bid.