Before I even try and recommend parts I would want to know what the upper end of your budget would be.
You could build a 2000$ bike that will get the job done or you could build a 7000$ bike with top of the line components.
Also you say you have always cycled on the road. Do you have any mountain bike experience? Also, are you talking about a true DH bike or an all-mountain/freeride bike? There is a big difference and that would determine what components should be used.
DBD:beer