[cadmonkey]

I have to agree with the previous posters - you're asking for a LOT for very little. A stepper machine won't do that kind of resolution without extreme gear reduction which limits speed hut that wouldn't matter for the size you're looking at. As for G-Code - that's something you're best to contact the manufacturer about. EMC can handle any standard G-Code thrown at it, and has some features in it that some other packages don't - I hand code most of my own work but I don't do, or should say I haven't done any relief milling. To even come close to the accuracy you want, regardless of stepper vs servo you're looking at HIGH DOLLAR ground screws and my bet is you'd need HIGH DOLLAR glass scales for position feedback. Even at short lengths they won't be cheap. EMC can handle all of that, but the mechanics are going to be pricey.

If you do manage to prove us wrong - do tell and brag to the heavens! (and patent anything that isn't already covered and then start manufacturing these cheap high precision units and selling them to payback your school loans).

[Stepper Monkey]

If you are bitten by the CNC bug, starting out by trying to make a mill from scratch is just not the way to go. Learn to be a machinist first. Without a CNC mill already at hand to make many of the parts required, a good manual lathe, and a few other more mundane goodies of a well stocked shop like a bandsaw and the like, it is going to be a pretty lost cause anyway (at least making one that performs better and/or is cheaper than just buying one).
Even more importantly, without experience and a background actually doing machining - not just for the actual manufacture of the machine but the design parameters you need to specify - designing a machine is not going to work. It is not at all trivial, even for engineers.
Even if you were to make a machine appear out of thin air right now to your imagined design, it still likely wouldn't do what you wanted it to as you likely don't have enough knowledge base now to even know what to specify in the design.

From what I see from the link you posted, nothing even close to what you specified may even be necessary though. Even a basic used Sherline or Taig setup will handily manage that sort of work without issue, either cutting the actual pieces or the molds for making them, and those mills are at the very bottom range for as cheap and light as CNC gets and still function adequately.
Try something like one of those and get your feet wet. Then determine what you need. That may be all you ever need, maybe not. When I needed more accuracy I eventually built my own machine using a beater stock Taig that held an order of magnitude less tolerance than that machine I built from it. If you really want to tackle building a machine you'll need to collect a lot of other tools that cost more than a small mill package anyway. Get your feet wet, learn, and once you collect the whole set maybe you'll be ready for a build. Or maybe you will find you won't need to and can be happy with modding the one you have. In any case, building first is not the answer.