I’m not sure of the minimal focus size, but it can be found by googling the laser and lens combination you want. There are a few really smart laser guys out there testing this stuff.
If you are just putting the laser on there you can go vey large 1Mx1M should be easy. If you start to get sag you can use mid-psan supports on the outer pieces. or start with stainless steel.
Best to just make 2 machines if you want to mill and use as a giant laser.
The laser needs about 50mm to focus, no reason to go any taller. As for milling no taller than necessary, a few inches is best 2-4.
Looks like you have it all on the list.
For the power supply you can get the 30A kit and use it to power both, if you really feel the need to. but really the 2 included power supplies would be fine.