There are some excellent 12V geared motors on eBay - you do need to find out the maximum RPM your project requires before ordering a motor, and also the torque required...
The motor you quote is 5280rpm, which with 12" long arms attached would be a lethal weapon (quite literally). That motor's specs seem very extreme (not what you want at all?) with a stall current of 115A. A motor controller for it will require over-current detection and shutdown. Any thoughts of using a simple single logic MOSFET to control it are doomed to expensive failure I think!
For 45 degree outswing on 12" arms I reckon you want about 55rpm. For 18" arms about 45rpm. Formula for centripetal accelaration is rw^2
(r = radius in metres, w = angular velocity in rad/s). For 45 degree outswing the centripetal acceleration is equal to that of gravity, ie about 10m/s^2