Fenders are an absolute must. And to the spare parts list, I'd add brake pads.
I'm fortunate that I don't have anywhere else to keep my bike other than the living room. In winter, this means it gets hosed down in the shower stall whenever I return. That keeps the salt and grit down to a dull roar.
My Trek Portland came through its first winter needing only rear cables, a chain and a new fender (which I'd broken). The year before, my low-end Giant hybrid needed a chain, brake pads several times and cables all around. The rims on it were really cheesy, and the brakes had worn them concave too. I don't hear a lot about that from others, so my assumption is that it's less of a problem on better bikes. Still, it's what drove my decision to switch to disc brakes for my commuting rig.
About clothing, living in Upstate all my life, and after having been a bus commuter for several years, I thought I had everything I needed for winter. Boy, was I wrong.
Everything I had was far too heavy and warm for cycling. It was great for standing in a snowdrift waiting for a bus, but not so great for activity. I found that in winter, I have the same problem as in summer--keeping cool. The first winter I muddled through with spring and autumn attire I already owned. Last year I went entirely with cycling-specific stuff and was much more comfortable. I haven't worn my official winter clothes in two years.