There's not much worry about overpowering the model. You'll do most of your flying at 1/2 to 3/4 throttle anyway, so what difference does it make what the max power is? I have no love for Evolution engines, but the word going around is that they are a bit better than they used to be. OS can't be beat though. As for the choice of a .55 or .65, either will power your trainer just fine. The .55 is made for .40 size planes that want some extra torque to turn big props. The .65 is a larger all around engine that can rev more with appropriate .60 size plane props. So a lot of the choice will depend on what you plane to fly next and whether or not you plan to keep your trainer for a long time. Quite a few guys move on to sport and aerobatic planes and then come back to their trainers and maybe do a few mods and start enjoying them again. You'll need an engine for it if you do that, so it won't really matter which one you choose now.