In your price range and market you can't be real picky about the brand and style, but you do want to be picky about fit and durability. Personally I like a rigid mtn bike fitted with slicks, but an older road bike can also work. I picked up a NOS Specialized Hardrock for $200 at my LBS once, and it's served me for 10 years. I just went in during the off season and asked what deals they had in my size. Again my preference is for fenders and a rack, so a frame with mounting eyelets is important to me. Fenders on a commuter are mandatory IMO.

The most important things are 1) fit, and 2) start riding.