Homebrew PCBs

It's not clear to me how much you're really going to be leveraging the existing software for the plotter though.  You might not have enough control over the plotter for things like (the equivalent of) feed rate, and you'll have to do something custom for managing the intensity (incl. on/off) of the laser.  For example, you'll probably have to spoof the pen up/down control to turn the laser on&off with your own electronics.

So in the end, you may just be using the mechanical parts and have to drive it yourself.  Nevertheless, having a working software/hardware setup that you could reverse-engineer would be A Good Thing (tm).