legalize, you're from boston? i am too.
which bike shop are you looking at?
for 500 dollars, you can totally get a good bike with front suspension, like the aforementioned hardrock or rockhopper.
if you really want disc brakes, you can always purchase them later. i mean, what kind of riding are you going to be doing? i'm sure regular rim brakes will be fine for the time being, considering you haven't biked in a while. maybe in a few months, you can upgrade to disc brakes, which would cost about 120 dollars for front and rear with 6 inch rotors.
anyway, some bike stores i recommend if you're in boston:
landry's on commonwealth ave
international bike store on brighton ave