There are several previous threads on this in this forum. The big questions are work holding (which you seem to have thought a bit about) and lifting more weight than is typically done (lifting all of the z axis instead of just part of it, basically). Both these things could be solved, but you'll probably end up trading floor space for higher cost of a motor and less workholding flexibility.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html