We use an Arduino UNO, not the Mega. It's important you purchase a Genuine UNO, not a knockoff or Counterfeit.
This one was determined to be Genuine: http://www.amazon.com/gp/product/B008GR ... 1_3&sr=8-3
Case for it. http://www.amazon.com/SunFounder-Enclos ... no+R3+case
Screw Shield for wiring to the UNO. http://www.amazon.com/LinkSprite-Screw- ... 2470955011
What size nema 23 steppers will you be using? That will determine the driver you will need. There is an Arduino UNO DB25 adapter if you want to connect a breakout board for a Gecko driver, or something similar. There is a 3 axis driver that plugs directly into the Arduino UNO, but it has a limitation of 2.5a per stepper. That is what we use on our Shapeoko. http://synthetos.myshopify.com/products/gshield-v5
Also what kind of mechanical drive will your machine have? Belt drive like our Shapeoko, or ball screws? Our PicSender does replace Mach3 and it will run fine with WinXP, but the memory will determine how large of a gcode file can be loaded and ran. The GRBL firmware that loads to the UNO, is only capable of 3 axis's. X,Y & Z, but outputs PWM with S commands to vary the laser's intensity also.