Ball screws

I am new to this group and have found it very informative. I have
some questions about using ball screws on a Enco style mill drill.
My intention is eventually convert it to cnc. First, of the people
who have replaced lead screws with ball screws on a mill drill, what
size, lead, and grade of screw did you select and why? Did you
calculate the maximum axial load the screw would be subject to during
milling or drilling? Where did you get the ball screws from and how
much were they (US dollars)? Was there any modifications you made to
the machine to allow you to fit and use ball screws? All input on
this subject as well as converting a mill drill to cnc will be
greatly appreciated.