If I were you, I would start with the battery voltage. Say it's 12V. Then, I would start looking for the 12V motors that use the smallest current at stall, perhaps under an amp. Use a SN754410 H-bridge to control them. Connect the output shaft of the motor's gear box to a threaded rod and use a nut on the rod to move the load. This way you have the most power out of the motor and the least speed. Make sure there is little friction on the threaded rod, grease it, align it, straighten it, etc.