Mark,
As has been said before, bike stuff probably isn't that critical-- a lot of people ride junker bikes with no special clothing. However, I'd avoid a used bike if you don't know much about bikes and aren't inclined/don't have the time to learn all about them quickly.

A relationship with a good shop (not all of them are good) is paramount. You probably won't figure out your preference for bike stuff right away. If you end up getting into bikes, it is almost given that you'll buy at least one more before you find the kind that you want. The same is true for other equipment. Don't spend too much money at once.

It may sound silly, but try not to stress out about this stuff. Trying hard to get bike commuting "right" can make it an unpleasant experience, when it has the potential to be much more enjoyable and healthy than driving. If you ride every day for a few weeks, it will become comfortable.