stepper motors and basic stamp

I have just about decided to use a basic stamp for my robot, and now I am trying to determine what kind of motor driver I need for my stepper
motors. I will be using two of these motors; one to raise and lower a platform weighing 15 to 20 pounds and the other motor to slide that same platform forwards and backwards. Both of these motors will be using a screw thread drive to accomplish this linear movement. I am not sure how powerful of a stepper motor I need as well as the type of driver/controller I need.
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