I'd recommend something from Synology in whatever fits your budget
https://www.synology.com/en-us/produ..._value%2Cds_xs
Thing is though running Plex directly on a NAS is fine if you never need to transcode, but if you do need it then you'll need to spend quite a bit to get a decent enough CPU. I'd suggest going cheaper on the NAS and then grabbing a used SFF desktop to use as the Plex server itself. Something like this is quite cheap and will outperform any CPU that you'll find in a NAS
https://www.ebay.ca/itm/233955781900...wAAOSwb5VfmLeA