always better to get a second set of rims...works out cheaper and better for the tires long run with the constant swapping on and off rims twice a year. and the time you spend waiting for the tires to be remounted and balanced each time.
tire rack sometimes has some great deals when they try to clear inventory.
rims - i'm partial to OEM even for winters. try kijiji? Not sure if the offset of the rims are different from the e60 generation. many used sets of rims from that MY. if they work get those. knock offs sometimes vary in quality and a cracked rim in the winter is far more troublesome than in summer. and we know how pot hole ridden the city becomes in the winter.