I think that you are on the right trail, except for the spindle, I recommend that you i'vest in an atc (automatic tool change) spindle, it's a game changer and a tool height setter. It means that you need compressed air as well, but that means you can use fog lubrication instead. I have a boxford mill which I've put the expert version of the controller that you show which has proved extraordinarily difficult to program, so much so that I bought the boxford software (£900) for the lathe which is a brilliant bit of kit.
I've also rebuilt a 3000x2000 router with a Cs labs csmio-ip controller and their SimCNC software which is pretty good (works with mach3/mach4) , I replaced the 7kw water cooled spindle with a second hand HSD 9kw atc spindle which I haven't finished the tool holder rack for yet so am using as a manual ATC which is a step change from the previous manual change. I had a great deal of trouble with emi in the control panel and ended up ripping all the wiring out and rebuilding using best practice shielded wiring and twisted pairs, some of those shielded as well, it's amazing the skills we aquire!
I've bought a few things from ratmotor who seem like a good company. The majority of stuff though I've bought on Ali express for the planned items and Amazon because when you need those m3x60 cap heads, you need them now!
Keep going