The only thing I can comment on is studded tires. They are awesome, you won't regret putting them on your winter commuter if you encounter ice. My winter commuter is a mountain bike with 26" wheels, disc brakes, and a derailleur, so I can't comment on the rest of your criteria. Belt drive and internal gear hub would be very nice, but getting all of features you listed in the price range you stated might be tough. Belt drive and IGH will probably put you close to your $800 mark, and you haven't even purchased a frame or anything else.