I'm interested to see what you come up with as I'm considering the same conversion, primarily because I already have charger, balancer and batteries for my T-rex 600.

the motor you've chosen seems good but may be a bit of an over kill, I'd be very careful with full power on that. My understanding is that 150 - 175 watts per pound is all that is needed for good aerobatics and mild 3D 200 watts for full 3D unlimited vertical.

Let me know how it goes I've thought about using the A123 batteries as well but the problem is where to mount them along with flight time. I'm looking for 15 min. and I think the number of batteries for A123 are more that the frame can handle if I do any real aerobatics.

Do you have a All up weight yet?

Hatch- depends on you building skills. What type of flying are you going to be doing? Since the wings were not designed to take the extra mass of the batteries they may need some attention. I would think that anywhere other than the engine and fuel areas for batteries other than lipo's may be too week and need to be reinforced.

for my Heli's I use the BEC but it's probably best to have an isolated battery for the servos one for redundancy two longer flight times