Well..... I'll start with that because it's a deep subject too, heh. ;-)
I don't have all the info off hand for that but I figured I'd throw something else out there... what about a smaller CNC router table? 2K might get you in the ballpark. I know it doesn't meet the "Learning to manually machine first" part but the work envelope on most would encompass the full 18" plus some extra.
Routers are very versitile as far as materials.. obviously they excel at wood but for robotics they can also do AL very handily and can also get a passing grade on a variety of plastics, rubbers, composites, etc.
Just food for thought.
Congrats on getting involved! My nephews school does that too... unfortunatley they live several states away.