I have an
Intel Core i5-12400. It was reasonably priced and available when I was building the TrueNAS server I use (and Plex runs on). It spends most of its life at idle.
Prior to my current setup I successfully ran a Plex server on a Raspberry Pi 4 w/ 4GB of RAM. I did not do any transcoding of files; all my files played back via
Direct Play or Direct Stream.
It is a hard question to answer without knowing what your usage pattern is going to be. My setup does almost zero transcoding. But if your library is going to be a bunch of 4K rips that'll are going to get crushed down to 480p on-the-fly to serve out to a phone on a crappy cell connection, then you might need plenty of power. I don't know how many streams an A310 can transcode on the fly. One for sure. Maybe two? But if you end up needing to transcode 5 streams on the fly more CPU is probably better than less.
If you're building a new box for this, any 10th gen or later Intel desktop CPU with 4 cores or more is going to be enough; probably more than enough. If this was your third Plex server build you might know enough about your usage to know that some old & crusty 2-core Pentium Gold CPU that you found on clearance for $20 is going to be more than enough. I'd guess a 14th gen Core i3-14100 (maybe even the low power T model) with 4 cores is probably plenty.
Plex has rough rules of thumb you can reference:
https://support.plex.tv/articles/201774043-what-kind-of-cpu-do-i-need-for-my-server/ Transcoding from 4K down to something smaller takes a lot of CPU, but in your case, quite a lot of that would be handled by the GPU. But then again, you can handle that with
multiple version support in Plex. In addition to your 4K rip, you can also store a smaller, transcoded version(s) of the movie in the same folder. Plex is supposed to pick the best version for the client.