My commuting started with a mtb for 5 years and then switched to a commuter style bike. The best part of a commuter bike is getting your head up so you can increase awareness.

The one item that will kill your interest in commuting are flats. So the first/only required place to spend money is on tires that can handle your route without flats. I mean zero flats. one a year is one too many.

Plenty of threads on this site describing the best tire / tube solution but ultimately your location / ride will determine the proper tire setup. Your local LBS, unless they are awesome at supporting commuters, may not have the knowledge to suggest the correct tire / tube setup.

Personally, on my mtb, I only found one tire that I could trust and that was the Specialized crossroads armadillo. - you have to be care full and buy the armadillo version. They are expensive but last in my experience up to 10,000 miles so they pay off in the long run. I only holed once with those tires - a 2 inch nail that pierced the sidewall. Its not a slick so you can use them on trail and road. (My commute with them was 70% trail 30% road but regulary rode on road and they dont give you the hum like a knobby tire. Ive also taken that bike with those tires on 100 mile rides on all road)

Its a fast tire and had no problem nearly matching my road bike speeds.