You may be dead set on the Rhino, but you may also want to look at the Dahon offerings for off-road. The Jetstream series should be price competitive with the Rhino... and it offers a foldable bike for those few times you need that.
If it's simply a matter of building up a Rhino with parts you already have... maybe get the frame, build it up and see if you like it, if you don't you can always go back to your Brommie or try something else. I wouldn't sell the Brommie until you're sure the Rhino (or something else) is the answer.
Good luck with your decision.
--sam