If you are looking for a business machine then you will need to pay business pricing unfortunately and of course there is a steep learning curve.

For preowned stuff, keep an eye on the classifieds in this forum. People usually sell good stuff, if they don't they get shot down in short order. The next would be ebay. It is variable in terms of quality but good machines usually have good descriptions. We all do our ebay searches now and then so if something the community feels would suit you, I am sure we will let you know

Traditionally, DIY CNC machines worked off parallel ports or serial ports depending on the manufacturer. Things have evolved though so you cannot find parallel or serial ports on laptops. The motion control would be done by the computer and it would send pulses to the stepper drivers. Depending on the make of the machine and if it comes with its own control software, your USB dongle or motion controller, effectively does the same thing. It receives data packages from the software that it converts to pulses that in turn tell the drivers to rotate the motors in this direction for so many steps.
In theory, any machine can work with USB if you add the required electronics.

Now regarding the Stepcraft: I looked at the videos and they all look good but the machine seems distinctly hobby. The stepper motors are smaller than the ones you get on the basic chinese ones (which does not fill me with confidence), the rolling elements are wheels on special aluminium extrusion and it has trapezoidal lead screws. It works and I am sure the support will be good (do your own research on this) but it will not be churning out the letters at any decent rate. I think 18mm py would really push the limits of this machine in terms of longevity.

Are you planning on small quantity production? I am thinking that it may be worth asking some waterjet cutters for a quotation. Unfortunately professional routing companies charge quite a bit, waterjet is faster, requires minimal setup and is so quick you can cut paper without it getting wet. Otherwise, laser, but laser on thick ply will leave charring.

Regards
George