I have received the DRV8825 drivers from China, and the CNC shield kit from New Zealand. Also, it has come to my attention that an earlier version of the shield was featured on Hackaday back in 2013! Anyway, fantastic quality and simple assembly process. The documentation was a bit vague, and the assembly instructions on Protoneer's website didn't quite match the actual product (newer revision of the PCB, website needs update), but it was simple enough to work out.
Setting up the software was bit trickier. I ended up install Grbl through the Arduino IDE method instead of burning a straight *.hex file, mostly out of convenience. I also created my own fork of the Grbl repository to document any modifications I make to the firmware.
Grbl is only half of the software, though, so that still left me with the PC/RasPi-side of things. For testing the hardware from Windows, I went with Candle, which is has a very intuitive interface. Once I get a bit farther with the mechanical parts, I plan to bring in GRBLWeb, which can even run on a Raspberry Pi 1 and provides a slick web UI for uploading G-code and such. Hopefully I'll be able to integrate a full Gerber>>G-code>>Grbl toolchain into a single UI. Writing an Altium plugin to automatically send Gerbers to be milled might be pushing it, but it's definitely a stretch goal.
While LPKF has been reasonably helpful, they refuse to give me any technical information on the stepper motors used in the Protomat C40. I contacted the company that manufactured the motors themselves, but was informed that they were a proprietary design manufactured for an OEM (i.e LPKF) so they couldn't share any technical information.
Tomorrow, I'm going to crack open the Protomat and start probing with an oscilloscope. The X, Y, and Z motors all work fine, so I'll get them moving and see if I can probe the operating voltages from there. Worst case scenario, I guess I could replace the steppers with something cheaper but better documented.
The motors are VEXTA C7391-9212, if anyone has any information. They are also marked as bipolar, so once I figure out a safe operating voltage then driving them should be no problem.