The best way to obtain high torque from a simple 12v DC motor probably is using some kind of speed reducer attached to the motor. It will "swap" velocity for torque.
A 3-phase motor doesn’t need to be stopped before reversing (of course it’s desirable, but not necessary). The magnetic field is able to stop the rotor and start the reversing procedure. Some cares must be taken.
Certainly it demands an extra current drawing, that is not higher than the peak...
I won't repeat the information already posted. Only one tip: in case of using 2 contactors, be carefull to provide an electrical or mechanical construction that guarantees that the 2 contactor will operate always one at a time.