I agree with steel frame suggestion. I recently bought a new $1500 commuter bike and opted for steel, and love it, this after some 20 years of thinking I'd never ride steel again. Don't feel like you need to get an aluminum bike, steel is a great frame material.

Road, hybrid, mountain, touring bike... all can work just fine as commuters. I'd personally recommend a touring bike as my #1 choice for a commuter, but most true touring bikes are probably over your $400 limit though.

Make sure that whatever you get can accept a rack. You may not want/need a rack for now, but sooner later you probably will (and then you'll kick yourself for not getting it sooner). Dual-suspension bikes generally don't accept racks very well (and any $400 dual suspension bike will be a real turd anyway, best avoided).

I personally recommend against used bikes, there's nothing like a new bike. But that's just me. If you get a used bike, make sure you thoroughly check it out or have someone else check it out. And ride it before you buy it, make sure the gearing is all smooth, everything's quiet, etc.