I am using the factory M3 18" Staggered wheels as my winter set. I just ordered the following from the tirerack which should work great since we get minimal snow. If we got more I would have gone narrower.
FRONT 245/40R-18 Bridgestone Blizzak LM-60 XL
REAR 265/35R-18 Bridgestone Blizzak LM-60 XL
Total cost for the full set of tires was less than $1,000 including shipping.
'07 Porsche GT3
'06 Honda S2000 #555 TTC
'08 BMW M3(sold)