If your money situation allows it, I'd say get a dedicated commuter. In the long term I think you'll be happier with that. More space for the add-ons bicycle commuters typically want, a less aggressive riding position with better visibility, greater versatility and tolerance of bad roads.

Whether you convert a used hardtail mountain bike (which mostly involves swapping the tires), or buy a used or new touring or cyclocross bike, or buy a purpose-built flat-bar commuter, is a personal decision. They're all perfectly good options. Whatever you get doesn't necessarily have to be steel, BTW. Aluminum works fine as well.