ENC coordinate systems

I'm working on setting up a benchtop mill with EMC. Can anybody
suggest resources for dxf to NC tool generation? Also, I'm interested
in determining coordinate systems for standard NC (EMC). (i.e. I'm
trying to figure out how I need to set up +x, +y, and +z to work with
standard NC output? Is z=0 essentially the endmill at it's lowest
setting, and z=3 three inches off of the surface? I'm still
figuring this out, but was hoping for some insight.

When I draw/generate my NC files (probably camexpert under linux with
some hand editing to better support Z axis movements), How do I set my
origin? I'm guessing I should match up 0,0 (x,y) in my drawing with
a matching 0,0 position for the mill. Any suggestions here
for linux based solutions would be great, I don't have windows).

For those familar with camexpert, do I need to draw "tool paths" to
account for the diameter of my cutting tool, (I didn't see a setting
on changing that), or is there a G code I can send to set the diameter
of the cutting tool? Will EMC move the tool exactly on the coordinates
in my
NC file (implying that I need to generate NC with a tool diameter
offset), or can I tell it the installed tool and have it account for
it. (I'm guessing that I need to give it the exact path the tools
should travel, and I (or my NC software) needs to account for the tool
travel.)

Each time I do a run, would I "locate" 0,0,0 for the mill, or can I use
home switches (are magnetic switches accurage enough, do I need to use
IR?).

Anyway, thanks for any help people may offer. This list is a great
resource!

murph