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.