Rich Whitehouse custom-built AVALANCHE from scratch if I'm not mistaken -- he later took his engine professional, which is pretty rad. If you have a lot of modeling knowledge, it'd probably be pretty neat to tinker around with his SDK; I couldn't make head or tails of it myself when I tried digging into it a little last year.
Looks like there's a pretty impressive load of documentation on 3drad though! I'm just getting my feet wet with Multimedia Fusion 2 myself, and I've found it's helpful to look at a commented custom engine someone's already built, even if it's way simpler than what the new user has in mind for their own project; a new user has to be taught how to think in terms of whatever language the utility uses before they can have real success.
