Compared to dual socket 10 core, even single 16-core Naples sounds much better choice.
Correction 20 cores 40 threads and they each have their own memory bus as well as well as cpu bus, not 100% shore what that server board is i picked but if its like this setup i wouldn't be surprised.
All you need is 2 pci-e slots for 4 GPU if you get two in one cards.
I've seen that an M.2 SSD performs much faster which is useful for caching data
SSD are 10x slower at writing then read given time just so you know making em not much better then old stile drives on this topic as well you can get a add on pci-e SSD cards if you want pure speed on this topic or just start going SSD in raid0
You do know as well two video cards are not 2x the speed, if you get +30% if your lucky and the more cards you add the less you get per card, 3 or 4 video cards is a wast of space and cash and by the time you get their down the road were your thinking of adding in more of the same theirs a faster single card out their that kills the pile in any case, twin video cards i can see for people that have cash to burn if you don't mind the down sides of having more then one card, more then that makes no scene.
All most starting to sound like your trying to cryptocurrency \ bitcoin mine or something.