That budget of yours puts you on the low end of the bike buying scale for a new bike. Do you know someone that is bike savvy? They could help you get a deal on a good used bike. You could get a gently used higher end MTB and still have money left for things like fenders, racks, locks, lights, bags and/or baskets. I rode a converted MTB for a few years as a commuter. Also how flat is your commute? Keep your eyes out for a Raleigh 3 speed. Quite often they can be picked up for very little. I would not want to be riding a brand new bike to the store unless you have a good lock and a place to lock it too.

Aaron