You're pretty much in the same spot I was in a couple years ago. When I started commuting I put some slick mountain bike tires on my '91 Park Pre Hammer and commuted on that for a few months before making a decision as to what exactly I wanted and also using it to learn basic maintenance.

If I were looking again right now and knowing what I have learned, I would be looking at this:

Motobecane Fantom Cross

It's a little over your budget but seems to be a cool bike for commuting and long rides.

As others have said, try riding what you have and get a feel for what you want to do.