I would suggest Cura for the 3D printing. and Slic3r for model slicing. You might want to consider one of the newer 32 bit boards instead of the Mega. BigTree makes some good boards but they are set up to drive nema 17 motors which in my opinion are too small for a CNC machine. I would size the machine and stepper motors for the job of CNC cutting based on my spindle and add the 3D printing as a extra. The reason is the machine has to be built stronger for the CNC function while the 3D printing has no real forces to deal with.