If that is still relevant for you (or for others who see this):

In my opinion, a strong CPU is nice to have for Plex today, but transcoding works better with a GPU like the NVidia P2000. Sure, the quality will always be better with CPU transcoding but for on the fly transcoding, which Plex offers for clients, the GPU is simply capable of transcoding more streams and faster without completely overloading the CPU (especially for 4K).

However, what else you need for a Plex server depends on the application. I personally have unRaid with 2x SSD cache and 15 HDDs. But of course it can also be smaller. With Windows (10? or Server) you need an SSD for the OS, which in my opinion should be at least TB, because Plex stores metadata, transcoding cache and database on the SSD. If you are stingy here, you will have problems when the SSD is full. Of course, you could also throw these things onto the HDD, but that of course costs performance and a higher power requirement, as the HDD then has to be started for every activity in the database.

And of course a USB HDD works well, especially for a small media library, but in the long run you can consider buying internal HDDs, possibly with redundancy via Flexraid or MS Storage Spaces.