Firstly the motor is rated at 1.2Nm - doesn't matter what voltage you use, torque depends only on current and current is
limited by winding overheating, so you need a 5:1 or 6:1 reduction.
The motor is nominally 23A (for 1.2Nm) but will easily pull hundreds of amps at stall if the power supply can provide it
so you want to choose a supply that will limit the current to 25 or 30A or so (ie a supply that recovers
gracefully from overload, rather than cutting out).
If you are running from lead-acid batteries you will need a current limiting device of some sort to prevent
damage (mechanical or electrical), so a motor controller that has current sensing and limiting would be
a good choice.
If you only need 14A or less continuous with intermittent peaks upto 30A then this is a possible controller:http://www.pololu.com/catalog/product/706
- but you'll need to work out the kind of load (torque)
is needs to produce to get an estimate of current.