900 total you'll be hard pressed to find something new. I don't know much about the cheaper bikes but i'd say Used would be your best bet probably. I'd look on ebay/pinkbike/ridemonkey for bikes like the Vagrant / Sirrocho, P-series, Morphine, Yakuza, etc. For 900 you're probably not going too great of a bike that can do Urban/DH.
You could try buying one of these two frames and building it up
The key would be to get a bike with good geometry and good tire clearance so you can run bigger tires in there.