Hi KT6G, cool project!
As for the other electronics items, you may want a spindle on/off controller. Your Chinese spindle may come with a control input so your PC can turn it on/off as needed, or otherwise it might require a power controller to turn the power on/off to the spindle drive box (VFD etc).
A z-axis tool setter might be good to consider also, it's easier to run wires and think about where it is all going before the machine gets finalised.
Have you tried cutting the alloy with your table saw and carbide blade yet? I've done it with a circular drop-saw no problems, with a squirt of WD40. For thicker pieces of alloy I cut on a bandsaw with a 6 TPI wood blade, alloy is a similar hardness to hardwood and that cuts it fine although not quickly.
As for suggestions on the design, I would think about specialising the design a little in that if you specifically aiming for cutting metals then make the machine a bit slower (choice of leadscrews/gearing) and much less Z travel and lower gantry height to improve rigidity. Looks like you already chose acme threads and supported rails so that seems a good choice for lower speeds and rigidity.
It would be nice to see a design or some picture, dimensions of the whole frame+gantry etc, if people can see your design they might offer suggestions along the way.
