Welcome to BHQ!
Buying a chassis and building it from the ground up is a great option. You would be able to set it up exactly how you want it right then. BUT, there are alot of great deals on complete dune/hill bikes right now. Look around and check prices. I bet you would be less money just buying a complete bike. Check out the "for sale" section here, there are always complete bikes, frames, motors, top ends...anything you need!
-Zach