Why on earth is the motor by itself 180 bucks (even if it's 4 poles, it's still gotta have something significant going on in there- Medusas are only about 120 IIRC for the 60mm versions and they're great motors!)? I wonder what diameter this "720 size" motor is (hopefully that nomenclature is not just a marketing scheme, and really denotes a larger-than-540 diameter size). If it's the same diameter as a Dewalt 700 motor....that'd be SICKENING torque.
In fact, that size motor would probably be ideal for these conversions; far less stress on the motor with such a large potential rotor.