With a $500 budget you'll be limited in choices. I would root around in classifieds, bike shops and craigs list for possibilities
- make sure I get a bike that fits. You could go to a LBS/big box store and see what sizes fit you.
- good wheelset
- I would say no suspension only for the fact that it would more than likely wear out fairly quick at that price point. I started with one. Shocks were shot in no time.
From there is all what you want