Assuming the MX400 is close enough to the 450 classes... Check this thread out for motor selection.

http://www.trextuning.com/3s-supertest.php

I just built my first kit after deciding to step up from the AXE CP.. the HDX450, and I' using the stock 3500kv motor and pinion that came with it, along with the Align ESC (yeah, the one with the heat sink on the OUTSIDE of the shrink tubing so it doesn't do much to dissipate heat.... LOL )

Since I didn't look at the MX series of helis I don't know much about them, but if you can use a 3500kv motor, I'd look at the JustGoFly motor they tested in the above post. Good Power/Flight Time ratings.

-Dave