I think you need to spell out your requirements for us before darting to solutions. Response between command and execution, sensors, motor specs, etc. all need to be identified. Identify the objective and a threshold for each requirement.
Will you be incorporating feedback sensors for positioning?
I think you'll do OK on this forum, but cnczone.com might have a lot more insight.
I can see where you may need real-time if you have some modeling in the loop to perform the next motion of the system. If you are just executing a set path in a loop-type method, then I don't see it being a requirement.
I'd recommend you dive into Arduino. It's easy and most of the coding work is probably done for you already. The ArduBlock software may even be sufficient for you.
Here are some basic Arduino links to give you an idea:
http://bildr.org/2012/11/big-easy-driver-arduino/
http://arduino.cc/en/Tutorial/MotorKnob
I would recommend looking at larger CNC-type stepper drivers - you can find some on eBay. You can use the Arduino board to send the command to the controller. All of these solutions require an external power supply.
If you are set using the NI solution, an external stepper driver shouldn't be too bad to hook up. You will need to generate a signal that can be interpreted by the driver. This goes back to your original methodology, and makes things seemingly simpler for you. If this is the case, you would have to find stepper drivers that can be controlled similar to the Arduino and build the code in LabVIEW.
I hope this helps you out.