Most of the people here are building routers, not mills. Routers are good for cutting wood and plastics, but not metals, since they lack the requisite rigidity to do a good job at it. And they don't have the precision you're looking for either. Mills of the size you're talking about tend to be made from cast iron and weigh tons. The mass helps damp vibrations that otherwise would produce a poor cut and break cutters.
My suggestion would be to find a used CNC knee mill with a bad or obsolete control and retrofit it with a new PC-based control system. Mills like this can sometimes be found quite cheaply, as the shops that own them need the floorspace and will often let them go for something close to their value as scrap metal. If you find something like this, it can have good ballscrews and other parts already mounted and ready to run, making it a lot easier than building something from scratch (even if you are set up to cast iron).
As for what electronic control system to use, wait until you've purchased the retrofit candidate to get all that. It may be that the motors it comes with can be re-used, which will save some money. In any case, it helps to know the loads you need to push around. If you're putting together a servo system, then you may need different equipment and software than if you're building a stepper system.
Andrew Werby
www.computersculpture.com