Stepper Motor
We used two identical stepper motors for our x and y axes. At 400 steps/revolution (.9 degrees/step), they are approximately twice the resolution of standard stepper motor. We used a smaller, 200 steps/revolution, stepper motor for our z axis.

Stepper Driver
High current steppers need high current drivers. We used the Pololu A4988 Driver breakout board. Even with heat-sinking and substantial forced-air cooling, we could not run them at anything near their 2A maximum.

These drivers are generally very robust as long as they are wired in correctly. They will shut off before thermal damage is an issue, and chopper voltage control encourages higher voltages for improved torque. The thermal cut-off point is nontrivial, however, and will make the steppers jitter as the temperature oscillates around the upper limit. That jittering causes a loud vibrating sound.

Power Supply
For basic operation, we needed a power supply that could supply the minimum current for our steppers and the minimum voltage for our drivers. Running a higher voltage through the coils of a motor leads to higher current, but was not harmful as long as we limited current to with their safe limits. Chopper drivers like our Polulu board are good at this sort of power management.