It's also a mountain bike conversion (Trek 6700). You already mentioned street tires-- keep 'em pumped up to their maximum pressure. I put a kevlar liner under mine-- haven't had a single flat tire since. That said, I don't leave home without a mini frame pump and tire kit.
Rather than fenders I installed front and rear racks (and a half fender/mud-guard on the front). I DO use panniers and like them a LOT more than my messenger bag or a back pack. I simply do not ride with anything on my back other than a hydration bag on long rides. Much more comfortable. I carry my work clothes, books and papers, lunch, tools, etc. in the panniers. As an added bonus, the bike is ready to tour.
Adjustable stem. I put this on to fine tune my riding position. I like it. I also use a set of comfortable bar ends to provide occasional change of hand positions.
Mirrors. I use a bar-end mirror and a flex mirror mounted on my glasses.
SPD pedals. I know you want to use your street shoes to ride, but once you become accustomed to cycling shoes and clipless pedals, you'll never want to tighten toe clips again. Seriously.
A Brooks leather saddle. I ride a B-17, basic black on my commuter bike. Best. Saddle. Ever. I've had the Brooks on that bike for about six years and it still looks almost new. It is well broken in but was more comfortable right out of the box than any other saddle I've ever ridden.
Headlight. I've been through several, currently using a low end Nite-Rider halogen. The nickle-metal hydride battery pack hangs from the top tube-- it doesn't preempt a bottle cage. Good headlights are ridiculously expensive. However, I ride on town streets and along a busy four lane highway, often in the dark, so skimping on lights is just not an option. A bright flashing tail light is necessary too.
Finally, I rarely (never) ride in street clothes-- spring for good quality cycling clothes that are matched for the weather you'll encounter, including inclement riding gear. I have winter tights that, paired with a polypro underlayer, will keep me warm on just about any day that it's safe to ride, at least in my relatively mild northern california climate, and the appropriate gear for warmer weather too. My commute is only about 8 miles each way, although I sometimes throw in an extra loop to the beach just because it's fun, but it's long enough that I don't want to deal with chafed bits from clothing seams all day. I change in my office and just hang the sweaty stuff up beside my desk to dry. It's bright, visible, and reflective, too.
on edit-- here's a pic, without the front panniers mounted (I don't use them very often).