A good brushless upgrade would be the Himax 2015-4100. I think you can use the gearbox that came with the plane but you can also get the motor/gearbox combo. The gearbox is aluminum. A couple guys in our club use this setup.
The 2500 would be overkill but would work, a little heavier and might tax your batteries unless they are 15C.
7.4 volt is a 2 cell pack and 11.1 volt is a 3 cell pack.