Get one of these:

MEC Flyer Tights (Men's) - Mountain Equipment Co-op. Free Shipping Available

MEC Adanac Tights (Men's) - Mountain Equipment Co-op. Free Shipping Available

MEC Drencher Pants (Men's) - Mountain Equipment Co-op. Free Shipping Available

Likely the Drencher or just some other waterproof pant over your work pants to keep the rain and grit off. Your commute is short. You'll likely be fine at a leisurely pace with fenders.

I have only had longer commutes and I found that I preferred running tights under most circumstances. They kept me warm enough in the rain and the sun off in the summer. When it was in the 2°C-4°C and soaking range I wore an older version of the Adanac tights. Kept me dryer therefore not hypothermic.