Fenders are great for year-round riding, and very nice for sloppy winter conditions. Your mountain bike should fit them fine, particularly if you ditch the knobbies for something more road-worthy (like a 1.5-1.7" smoother tire).
A rack is great if you also have a set of panniers or baskets. Otherwise it just sits there not doing much, other than holding a rear light and possibly acting as a partial fender.
Winter clothing you probably already have -- do a search on this forum, there's a million threads about it.
Biggest thing is just to make sure you have the tools to change a flat and find some way to carry an extra long sleeve t-shirt or jacket or something just in case you stop and get cooler for any reason. I nearly always have a sweatshirt in my front basket that I seldom use.....but just in case.