I'm a Network/R&D Engineer for a defence contractor.
I have a lot of outside interests, so a lot of projects on the back burner. The ridiculous mirrored pair of Plex servers was one of last years.
This year my big personal project is an autonomous vehicle (modified BMW i3):
depends on how big your library is and how many people use it.
not much choice in dual cpu boards for that form factor. In fact this was the only standard ATX (not extended) dual CPU board I could find that took modern CPUs
Plex library... 20 thousand movies and over a thousand TV Series (note... series not seasons)
I now have some 6TB drives in it as well as those listed.
The main array is ~65TB with a 512GB cache SSD.
The Plex database and image cache is split between two additional 512GB SSDs in pop-out bays in the expansion slots.
I have an additional server (Saturn) with an identical drive load (bought all drives in pairs) but just a single quad-core low voltage Xeon. Jupiter is in a rack in a datacenter with a redundant dedicated 1 gig circuit to the rest of the net and another dedicated 1gbit ethernet circuit to my city apartment (it's in the same development so I ran some cough "unofficial" fiber through the risers and trays in the car park). Saturn is in a rack in my parents place in the country and serves mainly as an off-site backup of Jupiter. It only has a 15/1 mbit DSL line serving it so streaming from Saturn is difficult.
I run rsync between the two boxes to ensure the bulk storage on Saturn is an exact copy of Jupiter. The Plex library is synced using Unison to ensure play counts are kept up to date.