If black ice is a regular risk on your route (and given your location, I strongly suspect it is) then a set of studded tires is an excellent investment. You don't need to go crazy with stud count--100 or so per tire is fine for street riding. I run a pair of Schwalbe Snow Studs (26 x 1.90, 100 studs/tire). I don't consider a second set of rims to be necessary, as studded tires should go on at the first risk of ice and stay on until spring.

As far as clothing goes, for a 6.5-mile one way commute, I wear winter gloves, put a headband under my helmet, and wear a windbreaker and windproof pants over my jersey and bib shorts. I'm fine down to 0 F (-17 C) in this gear. In below-freezing temperatures you don't need to worry about rain, so an outer windbreaker layer and maybe a couple thin inner layers for a bit of extra warmth should be fine.