Well, you carry more than I do on my commute to classes but I only travel about half the distance you do. I do suggest two panniers though. For one, I prefer to have the weight balanced (not that I notice it too much when the weight isn't); and two, I just like symmetry. I would always rather have two half full panniers than one packed pannier.

I would highly recommend getting as bright a light as you can get your hands on. Before I moved I rode a lot through pitch black back roads I was sure happy to have a super bright light. Now I commute through pretty dense city the whole way so I have downgraded my light; there isn't a section of road that doesn't have street lights around here.

My suggestions: two panniers, bright light.