Stepper selection

Hi all,
Need a little help here. I have a small mill (3 HP, 24"x7" bed, 12"x6" travel) and I am planning to convert it to CNC. Could someone guide me on the selection of stepper motors for it. What torque should I be looking for. Would it be ok if I use the existing lead screws or should I change them to ball screws too.

Regards,
Zafar


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