I'd look at a rapsiberry pi solution, cheap as chips and can add 4" colour touch screen for a £10
Ive been looking for the Pi but cant find many options, Any specific ones you know of?

As for 24v, The difference you will notice giving the nema's 24v is not like with nema 23's lol, you'll get 30% more torque but in the real world it will make almost no difference.
if i make the machine then the motors will be nema 23-24s, might have to go for separate drives any way.

I like the look of the Duet as its all in one but could be an issue if a section stops working. At least separates can be replaced