26in tires have far more options out there than any other size on the market. For winter commuting I just used whatever knobby tire I can get cheap unless you want to go the studded route. There are some "snow" tires out there that should work great in shallow snow but I doubt they would work well in deeper snow.
Fenders are a great idea. You can get some kool-stop salmon brakes pads if you want better we stopping power, they really do work MUCH better in the wet than any of the black colored pads I have tried.
Other than that, just making sure the bike is in good working order and that everything is properly lubed/greased and you will be just fine.