If theres enough snow on the ground it doesn't matter how good the derailleur is your probably gonna end up with only a few gears anyways, I'd say add the fenders and such and see how it goes from there. And yes make sure you have plenty of clearance with the fenders, in my experience fenders and knobbies dont really like each other from the get go. Also browse the winter bike forum, sixtyfiver is a great person to learn winter tricks from.