Hi Monty,
I went through this sometime ago, and IMO, don't bother - buy a CNC setup. Most 'manual' mills need so much work done to them to get them fit for CNC work, that the cost goes skyhigh. Usually the small mils have substantial axis backlash, so may need to be fitted with ballscrews - not cheap, because it may also need new bearings fitted, so machining work necessary. Then you need the steppers or servos to drive the axes - more cost, then add the cost of the controller system, and it starts getting crazy. A good CNC system isn't cheap, but still a better option than building one (unless you want to do it for fun!). A new CNC machine will come with a guarantee, you can be reasonably certain it will perform as specified, and you won't be involved in trying to build one for months (or years..:!).
That said, there is stuff on the web about building your own, so a bit of searching might help you work out what is to be done - unfortunately not always giving costs.