Would this setup work?

[simplechamp]

Yeah it would work, with the external BEC like you said. Might be worth considering getting a Mamba Max Pro. By the time you buy the Mamba Max and a Castle BEC that will be about $100. The Mamba Max Pro is only a bit more cash, has a built-in BEC that can handle up to 6S, and will have a 1-year warranty.