So you want the engine to surpass (or at least be equal to) UDK, but you're not aiming for professional quality? Sorry, but this sounds like a pipe dream to me.
Assassins Creed IV actually had ~1,000 people working on it, I'm guessing 3 was similar.
Well the core unit (the whole dev team) usually isn't more the 50 or so people. The ~1000 is marketing, sound engineers, grips, voice actors, and all the different teams that are used to do different things for the game that the core unit doesn't do.