I just built a new computer to use as a Plex server and Blue Iris surveillance system. I decided on an MSI PRO Series motherboard, an Intel i7-8700K, and 16GB of Corsair Vengeance 3000MHz RAM, and a Samsung 970 EVO 250GB NVMe M.2 SSD. I watched a few YouTube videos about NVIDIA GPU transcoding and realized I would like to keep that off the CPU. I did not like the limitation of two transcodes at once on the consumer Nvidia cards and I did not want to deal with hacked drivers to get around this. I therefore decided on the Nvidia Quadro P2000 by PNY, which was the cheapest workstation card without transcode limitations.
Since building this Plex server, I have been using it for a lot more than planned. I have been converting a lot of video to H.265 (pretty much around the clock) and watching movies on Plex. There have been no issues with buffering and this is with the CPU and GPU maxed out.
I would highly recommend the Nvidia Quadro GPU if you are a heavy Plex user. This was the best hardware component for my Plex server!