Ah, OK. You plan to have 7 motors, not 5 as stated in the original post. Neither RAMBO or Smoothieboard will support that.

What kind of reprap format are you referring to? Reprap i3 styles will often use 2 Z-motors and just parallel them onto the same driver, but IDK if that'd be a good idea for the heavier x-carriage like there'd be on a 16 inch platform. I'd likely agree that moving a 16-inch print bed in the y-axis is asking a lot for a single motor.

Sounds like this would be your first printer. I'm curious on what you plan to print. Anything using the full 16 inch platform could take days to print.

I'm not sure I can offer much help. Good luck!