For longer commutes, a touring or modern cyclo-cross/tourer are ideal. They are fast on the road, and suitable for light trails, can take rack and fenders.
Check out icebike.com to see how far into winter you can ride.
If you want to reduce the initial spend, then some 1.5" slicks on your current bike will do the job. I wouldn't recomend a back-pack on longer rides. Let the bike support all the load on a rack.