You can get an 05 rockhopper for 500 bucks retail...better bike than the hardrock and more upgradeable. You dont want to pay 500 bucks for a bike with disc brakes. Usually on a 500 dollar bike with disc brakes theyve taken away from something else...considering that good disc brakes are from around 300 to 500 bucks get v brakes then upgrade later. Get a better bike and then make it better. Since I started this whole thing I've realized that if you want a really nice bike you have to empty all your pockets. Just empty them lol (kidding)
anyway research is your best weapon
goodluck