Milling machine endstop hardware selection?

CrashOverride? Hackers, the movie?
That depends, is the machine going to use coolant?
If yes, most probably mechanical fully sealed or inductive ones, although inductive ones will have to be protected from debris.

I use inductive, buried in the ways.

photos.app.goo.gl/bPETVs7sDLAK8a9r9 (during wiring)

And similarly on my lathe:

photos.app.goo.gl/yVMkdVpRTrHHyCvm6  (not normally visible, the ball nut has been disconnected for the photo)

Targets are shallow drilled holes. 

Note that you need the "shielded" type for this to work, with the screw-threaded case reaching all the way to the end. The other type (with more plastic visible) will be triggered by the hole that they are in.