Well,
Then I'm not sure I have any advice for you. We've built two desert tracks now modeled after king of the hammers, and while we've tried all kinds of trucks, U4 seems to be the most fun. We also went with a few DB48's, but the Lasernuts made short work of them and they were sold off.
When you say sky is the limit, are you talking about tens of thousands of dollars to invest, or just pushing some dirt around with a front loader.? If you can afford to have 2 to 4 ton rocks moved in place, well then everything changes.