The 20 ga 7 strand will be fine. Your not going around any corners in the wing so the stiffness shouldn't matter.. Try to get yellow black and red (or brown hi-tec color) wire, and you can twirle all 3 of them in a drill so the lead is like the old futaba twisted leads.

The 1/3 scale beaver is a little to big for my liking. I think 1/4 scale would be the biggest I could afford and fit in my house..You gotta build its own hangar in your yard for anything bigger... That Unionville 1/4 scale one seems interesting...