I don't really have a winter bike. The steel roadie usually ends up in the trainer, the carbon roadie gets ridden outside but with SPDs, not Speedplays, and the aluminum commuter gets ridden in all sorts of crap 'cuz it has fat tires and fenders. When it snows, the rigid 29er with fatty-fat tires comes out to play (or the XC skis!).

I think you're on the right track with fenders, studded tires if it's going to be icy, and fat tires for anything else. Single speed would definitely be simpler and cheaper, once you nail down the right gear ratio. I would want either cantilevers or disc brakes for the extra clearance and stopping power.

What are you thinking for a frame? A cyclocross or 29er frame set up with dirt drops?