I agree that snow tires are the only way to go with a Mercedes. I bought a set of 15 hole rims for $50 on ebay and keep a mounted set of snow tires for winter.
Another BIG factor for snow traction is tread depth. All season tires will slip like crazy if they have less than 6/32" tread. Same goes for snow tires. Be aware if buying used snow tires that some of the best names like Blizzak and Graspic have a special tread compound that is capped on the tread. A tire with 8/32" or 9/32" tread may only be good for one more season. New snow tires have approximately 12/32" of tread.