CAD/CAM for Education

I teach CAD at the high school level, and am considering adding a
CAM unit. I have two questions:
Does anyone have suggestions on an affordable bench-top size
mill
that will survive in an classroom environment? I would like
something more robust than the little toy-size units that are
marketed to educational institutions, with a work envelope at least
4 x 6 x 12 inches or so. I don't have room for, nor can afford, a
full size machine.
Also, any suggestions on affordable software to convert files
from
AutoCAD, Inventor, or maybe Rhino to G-code?
Doug