As the shocks travel, they take energy away from your drivetrain. They also add weight to the bike, and really do little for the ride on pavement. You can avoid most any obstacle that would make shocks even mildly usefull by the simple expedient of paying attention.

Fenders are great if you intend to ride in the wet, or if you live in a wet climate, where such rides are unavoidable. Otherwise, they add some wind resistance to the ride. I'm setting up a single bike with them, especially after last year, where it seemed to rain here every other day.

Road bikes are, in general, quicker than MTBs, but most newer riders prefer the upright position an MTB affords. Vision is very important on a commuter-- the obstacles you encounter move around at random and often seem to intentionally try to hit you (mostly because said obstacles are driven by idiots, but that's another matter). If you're trying commuting as a newer rider, I'd recommend an upright position (MTB over road). If you can find an older, well made steel frame, it'll last forever with some care. Early MTBs make nice commuters. I ride road bikes because, well, I've been riding them forever.

I like a slightly wider tire on my roadies; I've got a mix of 25 and 28cs, as well as a single bike set up with 27 x 1 3/8. (The trek 850 will have 26 x 1.25). I don't think I'd commute on 20s.