For a business, it would be extremely hard to cost justify refitting an old mill unless the stars line up just right... That would be a person with the right skill set that currently doesn't have any paying work to do. An example would be your electrical sytems person if all your manufacturing lines are down/slow.There is a LOT labor involved in refitting a machine. And it needs to be a labor of love.
That said, a VFD for the spindle is a real good idea. Gives you variable spindle speed.
If I were you, I'd look to the gently used market in something like a Haas minimill or VF3.