I think that any bike that has fenders/lights and makes you smile is a "commuter".

Even a longish commute is not a terribly demanding ride, as rides go. Would you ride that ____ for an hour on the weekend? Then you could ride it on a commute.

I've commuted on a vintage mountain bike with slicks, rack, pannier, full fenders. And I've commuted on a vintage racebike with Raceblades. They both made me smile and were fast enough to pass just about every cyclist on my route.

So go over to C&V, find a couple makes and models that trip your trigger, and start learning and looking. Winter is a good time to buy a bike, and if you watch Craigslist long enough and act fast enough when you see something good, you can pick up a "very" nice older bike for $200-300.