Definitely mudguards. A rack and a rack bag are more comfortable than a backpack - no need for panniers unless you want to do your shopping on the way home. I had 32 sets of traffic lights on my commute into Glasgow - wish they had disc brakes available for road bikes back then, rims took a pasting. A good solid lock, preferably with a chain or cable to go through the front wheel as well, is a must unless you've got a secure lockup inside the building. Good tyres too - I used Conti GP 4 Season (and still do). Nothing worse than a puncture on a cold, dark, wet morning.