For $400 you aren't going to get an amazing bike.

As it has been said the local bike shop will be able to recommend you a decent $400 bike for riding to work. It may not be especially suitable for trails though. You should also steer away from huge brands as they will be more expensive as you are paying for name rather than the bike.

You should probably start with a fully set up bike as you can always change the parts once you get a bit more experienced. Just aim to get a good bike frame and work up from there!