You can use your existing control setup with the new hardware. Its probably as robust or more so than what they are offering which is effectively 3D printer electronics and drivers, modular and cheap but limited in output for a larger machine.
Looking at that design, you will probably be disappointed by the lack of rigidity compared to what you have now. I don't know how fine a detail you do now but the larger machine will not hold nearly the same tolerances as the small New Hermes.
My experience with things like this are that they are a great introduction to the hobby but you either move "up" or or give up from machines like that - they've made too many compromises in terms of cost that the performance just isn't there. For example - EMT is not known for its straightness. You'll probably waste countless hours chasing the imperceptible curve in each piece you've cut, trying to get the machine to work just right. This was my experience with a 3D printer of similar design, using much shorter smooth shafts that also had small curvature to them.
Hopefully you can find a youtube video of somebody who has built one, but IMHO that design is too flimsy to be worth 120 hours pf printing, plus assembly etc.
Mike