If the bike already has disc brakes and wheels, that's great. They aren't as important as fenders or studded tires, though.
Full plastic fenders will keep you driest. If your bike has fender mounts these will work great. Many full fenders can be rigged up on frames that don't have the special eyelets.
Bike Nashbar has a deal on great fenders for $25.
http://www.nashbar.com/profile.cfm?c...All%20Products