If you can get away with a single speed, why not?
Any frame material will work.
Coaster would be fine, but I would go disc simply for hand control.
Upright/flat bars for dress clothes commuting. At least enough that your tie hangs mostly straight down your chest.
Definitely full fenders and chainguard. Single speed makes this easier.

Consider your saddle carefully. The rear/crotch area of dress pants are no match for regular cycling duty (although four miles/day isn't much). A slick saddle will be easier on the fabric of your pants. Shoes or a shoe change is another consideration because no matter the fender coverage, your feet will catch a lot of grime in anything but the nicest weather.

A 2 mile commute could be walked comfortably in about 30 minutes (even if you're pushing a bike). I'd not carry anything more than a folding lock -- no tools, tubes, pump, etc.. Minimalist to the extreme.


-Kedosto