For your budget, think used bike and if you don't know a lot about how to look at a used bike I would .

The important thing for a commuter are you don't want to skimp on frame integrity and tires. For gearing - it all depends on how many hills you have to climb - I'd look for any grade Shimano in good repair.

You'll want to leave room in your budget to invest in lights, reflective gear, whatever you need to get the right clothes and computer to your work and a helmet. Finally, if you're considering a specific bike, google it. You'll find plenty of threads about recalls and positive or negative experiences.

Glad you're joining us on the road to work :-)