Use any ball bearing .46 and run an APC 11x5 or 11x6 prop on it...it doesn't get much better than that! When you start with the forward-most CG it will be a puppydog, go to the intermediate CG you'll have an advanced trainer...go to the rear CG and it'll do stuff you never thought it would
Build it strong, build it light but more than anything build it straight!