You could call Aeroworks and ask them; they are nice guys, and helpful. It would also pay you to run a search on Aeroworks 300L Profile ARF.
Their manual says CG on the main spar; RCU users report further back locations.
Their manual says for Low, High and 3D rates:
ailerons 20 25 35
elevator 12 25 35 to 45
rudder 15 25 full (many RCU users trim the elevator to increase rudder throw)
Lots of expo on highest rates.
For the engine, I think it is a matter of what balances out right, and provides enough power to suit you; people speak well of the OS 160fx on this airplane, unless they have nose-heavy problems.
Happy harriers!