I like the setup, but have questions myself on the video choice, for VM and Plex use.
- I know Intel makes drivers for virtualization, but I'm uncertain how far the ARC series is supported. I see giganode has a "Intel i915-SR-IOV" App on the main screen, being uploaded recently, and it clearly does NOT support Arc boards.
- I'd verify on the Plex forums on Arc support as well, to get an idea on compatibility and capability. I know the integrated graphics support on newer Intel chips lags a bit, but I'm not sure on the Arc series itself suffers from the same.
As for the SSD's, I don't know that you need two sets, but I am no expert. Having a 2TB mirrored set, I feel, is more than enough. The real storage-hog are your plex videos, which will reside on your disks, so everything else you noted should leave you more than enough on the WD Black's you mentioned. Would suggest doing more research here...
The processor, for me, was the biggest catch when picking a setup. Intel desktop proc's are very, very limited to PCIe lanes used for expansion. If you ever expect to install other boards, besides the video card, like SAS, 10Gbe, etc., you'd need to have a workstation or server motherboard that supports a Xeon (Intel) or Epyc (AMD). Several benefits open up here, like IPMI, that allows you to KVM into the system, all the way down to the BIOS/UEFI settings from a remote location, even if its only in the other room. Not having a keyboard, mouse, monitor is nice too.