I have a 2006 SLK 350. I'm guessing the 350 is heavier. Don't know if that would actually be good for giving me more traction or bad for more inertia in a slide. In any case, I use winter tires and wheels. I did finally manage to get stuck in snow this last winter. About 6 inches fell in the parking lot where we were and I couldn't get traction to get out without a push from the wifey and someone else who probably thought I was nuts.
SLK 280 in Uk Winter is OK until temperatures hit 6C the car is very poor at temperatures below this. The back end slips out all the time.
I drove behind a standard 1300cc hatchback last year Dry conditions at 6C and the car in front was fine but no way could I catch it, my car was so tail happy on smooth / worn bits of road.
Continental Supergrip 3's
As others have said Winter Tyres are required.
As a note : the Handbook that came with the car front page leaflet advises not to use car below 6C.
Hope this helps. Continental Supergrip 2's are slightly better believe it or not.