Keep the $600 and find a $150 used road bike on eBay. Spend $50 on a tune-up once you get it and save the remaining $400 for the accessories you'll need that you don't know about yet today.

After riding the $150 road bike for a year, come back to these forums and tell someone else your success story and stop someone else from spending $600 on a used bike for commuting.

Trust me.
(if only someone had given me this advice, or more accurately, if only i had listened. )