Maybe the Pi isn't the way to go, but my thought process was to create a g-code interpreter to drive the stepper drivers etc. and provide a real-time display of the process. Sort of MACH3 in a matchbox... (or should that be EMC2 in a matchbox)... anyway, a cut down version. No frills, no bells n whistles, just motion control and some graphics. I don't think the other boards suggested can do this (well of course with a multitude of added perpherals maybe). Big ask? maybe.

I/O isnt an issue, already sketched some ideas for a 16 port TTL compatible I/O module that uses a minimal # of GPIO and will be fast enough for this purpose (a few uS switching time and some bit-banging software) with an RTC as well. Also looking at using the I2C interface for the same, albeit slower...

So far just knocked up a simple I/O connector to get to the breadboard easily.