I would just say this: Cyclon, as 3DownLow mentioned is the obvious choice. You will not believe the difference between an outrunner like the AXI and the way the Cyclons deliver their watts People who have not tried should not try to talk you away from it... But I guarantee you will be amazed and pleased.

If you pick the Elite 18, and power it with 6S - 7S A123 it will nearly rip your plane apart
My first meeting with Elite`s lead to investing in more Cyclon!
But it can`t be "backmounted"... And large diameter propeller is needed to get all there is.
Gearbox maintenance: if the gear isn`t dry after the two first checks and relubs the maintenance intervals can be prolonged.