I think that a NAS Killer build might work for you as long as you don’t need to do too many Plex transcodes at one time.
If you are looking to have a box that can function as both a Plex server and a NAS, you may also want to consider an OTiS build. It would allow you to take advantage of the Intel Quicksync technology to do hardware transcoding in Plex, but can also do NAS stuff (like run some Docker containers, VMs, serve files to other computers, etc). For more information please give the following forum thread a read.
I hope that helps