Rebuilding Enco Mill Drill

I'm rebuilding an Enco Mill Drill (Model 91007) and plan on adding
three axis CNC. It's an older machine but in good shape. I have
three VEXTA (Oriental Motor) 2-phase steppers, (4A, .97 Ohm,
1.8d/step ). Each has a UDK2160A Driver and SC8800 Controller.

I'm pretty comfotable with the mechanical mods but could use help on
electrical/software side. The questions that I have at this point
are: 1) are these motors suitable for the XYZ axes 2) What's the best
way to get a computer talking the SC8800 3) What's the best CAM
software to use to get started (I use Acad and Solidworks for CAD)?

Also, the Enco has a 2HP 1-phase motor (1720 RPM) and a speed range
of 140 to 2440 RPM by changing belt pulleys (yes, it's always set on
one speed regardless of the task). I'd like to upgrade the spindle
drive to include easier and broader speed range.

I'm thinking of building a 5HP rotary converter and installing a 3-
phase motor with variable frequency drive. I'm not sure what happens
to the torque at low speeds using the VFD. I'm also not sure how
fast I can push the spindle; it has an R-8 collet and draw bar and
uses single row conical roller bearings. Is this the best approach
and how fast can I go?

Finally, I'm thinking of putting the Z axis on the down-feed knob
rather than directly on the quill. This means the downfeed must be
engaged and travel will be limited but it seems a lot simpler than
connecting to the quill. Is this a short-cut that I'll regret later?

Any comments or suggestions would be most welcome!