Hello 9fingers - I have a similar mill to this which I purchased about 10 years ago, and I cam considering a CNC conversion for it. Well perhaps a bit more than consider as I have purchased the drives for the stepper motors already :-)
Anyway, I was wondering why you chose the path of modifying the knee, with the need for the gas strut etc, rather than just modifying the quill. This is the route I am thinking of taking. My logic is that it appears simpler and I don't think I need z travel much large than 100mm or so.
Alan