Is building the mill itself as far as your thoughts have taken you, or have you considered the potential use of the thing? For heavy metal cutting, you need maximum mass and rigidity; for lighter cuts in softer materials it's less important. What size parts do you want to make? From what materials? Will this have a 4th axis, 5th axis, or just 3 axes? I imagine there are plenty of disasters to be avoided, but starting with a solid plan of action will help with that.
I'm not sure that your first consideration should be the power supply you happen to have on hand. I'd say start from first principles, and get the power supply you actually need to accomplish the work you want to do (unless by "power supply" you're meaning your mains current, which you'll either have to live with or move.)
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