From your description, it sounds like you're planning to use 3 rails per side of your machine. This won't be easy to get set up; it's hard enough to align 2 rails perfectly.
Wait until your frame is finished before shopping for electronics and motors. These things tend to evolve as they're being built, and you don't want to be stuck with stuff you can't use. The extra time will allow you to decipher the "Greek" and learn about other factors besides holding torque - inductance, for instance,, is one which can severely limit performance.
If you do a great job on this, you might be able to get acceptable results on aluminum, although you'd have better luck with a fixed bridge/moving table design rather than a moving gantry.. Steel is much less likely, and stainless milling just isn't going to happen on a gantry router. That spindle is all wrong for materials like that .- too fast and not enough torque.