You are correct in some remarks, I have proper reading issue :- ).
You write Asus Z10 PA-D8, I just assumed you meant WS version. You're right, this one is regular ATX, will fit anywhere.
I suggested EVGA because they make the cheapest dual cpu compatible PSUs. Seasonic, and Seasonic-made Corsairs are bit more expensive. But yes, of course preferable.
Still, the Arc vs Define stands. I have 8 Define cases at home and they are utterly silent. I built the Arc with identical components and the difference is far too big. (there are even after-market silencing kits for Arc, go figure ). The only time Arc is bearable is running at <500 rpm for intake fans. Or 5V for the default Fractal 1000rpm fans the case comes with. The Define, on other hand, is still silent at 800 +/- rpm, or 7V for the default fans.
ECC wasn't really needed for previous generation, non-ECC work great on my LGA 2011-1 workstations ( 2670v1; 2680v2 ), in both single and dual-cpu configuration.
For LGA 2011-3, non-ECC only works with single-cpu configuration. Not only is this stated on the website, but, I tested it :- ).