1st Pick: Clint Mansell (Moon, Black Swan, Mass Effect 3), or
2nd Pick: John Murphy (Sunshine, 28 Days Later, Kick-chull), or
3rd Pick: Paul Leonard-Morgan (Limitless, Dredd).
All three have a very futurist way of scoring and aren't limited to doing it all themselves (i.e. pulling existing popular music into the scene) for added realism and/or depth. If they could afford better than either of these three then the next natural choice would be Hans Zimmer, but I'm not sure if his tone would be too epic for a Mistborn series.
I focused mainly on the composers, but for sound design it's really hard to narrow down. I like the idea of adding musical cues to their powers and providing the characters with unique sound-scapes.