Since you're talking about machining metals, I'm assuming you want to build a mill, not a router, but the other components (the extruded aluminum frame, the thin aluminum plates, the high-speed spindle) sound more like a router. Do you have a design drawn up? Does it have a moving gantry, a fixed "bridge", or a "C"-shaped frame with moving table? When you talk about "the highest accuracy and repeatablilty possible" you need to qualify that with your budget - possible for a $1000 build, a $10,000 build or a $100,000 build? Ball screws, for instance vary a lot in quality, the C7 Chinese ball screws are what many people here use, but they may not meet your expectations. I'm not sure that you can build a spindle with belts and pulleys that spins at 40,000 RPM; I've certainly never seen one. I think the whole thing would be likely to fall apart on the way up to that speed, unless you spent a lot of time dynamically balancing everything. I'm not sure what you mean by "classic" Chinese CNC controllers, but if that's a euphemism for "dirt cheap but they seem to be selling a lot of them" you might find they don't actually work.
As for your questions:
Precision-ground ball screws cost considerably more than rolled ones. If high precision is more important to you than low cost, buy them.
Reputable brands of linear profile rails include SKF, Misumi, and THK.
There are companies that sell complete motion control solutions, but few of them are focused on 23-frame stepper motors. Usually they are AC servo systems. But feel free to take a look at the companies listed on my site for various alternatives: https://computersculpture.com/links#Miscellaneous
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