It would certainly be easier to put a new control system on that old mill than to build a mill from scratch. If the ball screws are still in good shape, that would be a plus, as would the ability to use the old motors and power supply. The main things that you can check are the condition of the ways and spindle. Sometimes a mill has been used for a single operation, over and over every day for years. This wears out part of the ways. Try locking it down at the ends of travel, then see if the lockdown screws need more travel to lock it in the middle. If you can run the spindle, see how much runout it shows. If it doesn't run, see if it deflects when you push on it.