$500 is a really limited budget...
I would not choose disc brakes at that price point, I would go with v-brakes and change pads to someting from kool-stop. Trek, Specialized, Jamis, should all have a bike in that range, but its going to be low end. Used might be an option if it was used lightly and not abused. Around here you can get a LOT more of a bike for $500 if you search craigslist... just check frame over REALLY well for cracks if its aluminum.