I sucessfully converted my X2 mill to CNC and have been more than happy whith the result. Did mine the hard way without the add on kit. Mine has a ball screw on the Z axis and revised square thread leads on the X and Y axis. I replaced the original (poor) acme lead screws with square thread versions cut on my lathe. I used bronze anti-backlash nuts, also cut on the lathe. It was a lot of trouble and the kit is probably the better way to go.
I use Autocad Vetric2D and Mach3 with the Newfangled wizards. Vetric and Mach3 are great value and very user friendly.
My motor choice was similar to what you are considering but the motor control is all home grown. I have microstepping drives on all 4 axis, PIC based motor controllers and a home built 40VDC regulated power supply.
My latest addition has been a hi-speed spindle which mounts parallel to the main spindle and is driven from the top of the main spindle via a slotted belt pulley. The pulleys were cut using the 4th axis and the Mach3 wizards. It runs 10,500 RPM and is ESSENTIAL for cutting aluminium with small endmills.
This set up gives reliable torque and power with low TIR even on long and complex cuts. I use ER11 collets on the high speed spindle and ER32 on the main spindle. This addition has vastly improved the mills capability, better finish and much higher cutting speeds.
You can do a lot with this setup and it is vastly more capable than a manual mill. Photo attached, I hope.
Hope this helps,
Graham H, Sydney OZ