a cheap 29r would be ok, redline monocog... kona unit.

The issue with using a MTB for commuting is that a lot of manufacturers don't consider racks or fenders. If you get a mtb be sure it has eyelets on the fork and frame for mounting fenders and a rack(not unworkable but very annoying). If you go with a 26" modern mtb it will have short chainstays and your feet may hit the pannier bags. You'll also have to fork out a bit of cash for the set of slick tires since mtb will come with knobby tires.

A hybrid bicycle sounds more like what you want if you're not going to be doing offroading and need the heavy components. It has gearing which is ok for hills too.

bikesdirect stuff is usually is fine, you'll have to be confident to tinker with the bicycle though.