Craig,
OK, so I think I'm getting even closer. Let me validate a few things. I'm a materials guy, not an electronics guy, but I can learn and have done my share of soldering and electronics troubleshooting.

So, big picture, my current old computer is actually converting the g-code and creating the signals needed by the stepper motor controllers, etc. This is passed through the parallel port wires to the bob and distributed to the relevant controllers and devices.

To switch to USB or ethernet for use on a laptop, we move the signal creation to a new device, the suggested motion controllers. This receives the commands and creates the relevant signals (thus less noise). These signals are then passed on to a bob and the rest is the same.

Separately, to get spindle control, you are proposing converting the PWM signal that my bob is capable of into a 0-9v isolated signal that the KBMM-125 can use instead of the potentiometer. I was trying to purchase a bob capable of the 0-10v output, but this approach makes sense.

So, unless I'm missing something, I can do this in stages. Get the spindle going now using my old computer, then get an ethernet motion controller and connect it to my current bob using a ribbon to parallel cable.

Did I get this all correct?

So, I'm sure I'm missing something here, but if I were to purchase the ESS-MB3, that has an isolated 0-10v output that I could just wire into P1 and P2 on the KBB? I know the KBB wants 0-9v, but it seems I should be able to set the scale in Mach3 to not go to 10v.

Thanks again for all of your insights!
Mark