Popping it to say I had a productive afternoon. Got everything up and running, sawdust a flyin’!
If your not planning on using your MPCNC for 3D printing, I see no reason to go the Arduino Mega/Ramps/Marlin approach.
Save yourself some dough and pickup an Arduino Uno, a CNC shield 3.0 (or higher), stepper drivers (~$25 total). Then load grbl on it.
I have found that it is more user friendly to use firmware and work flow that is native to milling/cutting.