3D model

I've snooped in this group for a while, hoping some day to get a CNC
unit. I've found a friend who has a Rong Fu 7x22 with BobCad 20, and
a middle school student who has got a project that might work on the
mill, but we are a little unsure how to proceed. The project is to
make a 3D model of a transect of our valley. I am more a geographic
information (GIS) guy and can provide either a set of contours lines
or polygons, or a gridded surface, or probably even a set of 3D points.
The question for the group is which is the best way to get this GIS
information into BobCad? The GIS stuff is probably best represented
in UTM (meters N of equator and meters E of a special meridian and
meters above sea level). Should we then rescale this in BobCad, or
before we get there? Does BobCad import Dxf, as I think I can export
in that format? How do we ensure that the naturally sharp ravines etc
are represented in a way that allows for tool dia, and won't preempt
the project?
I think we are looking at shrinking a 10 mile distance to the 20+ inch
excursion of the mill, and exaggerating the Z axis to about a 2 inch
excursion (it is maybe 1800 ft total in real life)
Thanks for any help.