There are several people using the Duet with external stepper motors. I added an option to the M569 command in my firmware fork several versions ago to make this easier, by allowing you to switch the drivers for particular motors from the internal drivers on the Duet to the ones on the expansion connector. One issue you could face is that the step pulse width generated by the Duet might be too short for your external driver, but I haven't heard of anyone having this problem.
You don't necessarily need an external driver for a more powerful motor. If you choose a Nema 23 motor with a rated current in the range 1.5 to 1.8A and you use 24 to 30V power, the Duet should be able to drive that, as long as you are not looking for very high speeds.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
