Having seen it recommended here before and having done it myself I'd suggest getting a good used hardtail/fork mtb off of CL or from a pawnshop. Having experience w/BMX you know the difference between **** and shinola, so you'll get a decent bike for around 200 I'd venture. That'll give you plenty of room for fenders/rack/bag/lights/tools/tubes/patchkit/tirelevers, etc. and all the other accessories one needs for ease of commuting.

A Specialized Hardrock is a pretty solid choice. Stay away from full suspension or forks as they slow one down. Older hybrids are cheap, too and faster than mtbs in general because of the 700 wheels. http://www.bikeisland.com has a real nice Park multi tool for 20.00 w/no shipping. Also, a Topeak Road Morph w/gauge frame pump for 35.00...again no tax, no shipping. Good luck, enjoy your search and ride the crap out of whatever you choose.