I use a similar one of these (different io panel and case) for OPNSense and it’s been a delight. Not sure how well it would handle Plex but if you don’t need to transcode the files it should be fine. I bought one without ram, os, and storage for $150 last year. That listing isn’t available anymore.
Yes, the seller was Topton Computer Store. No complaints from me on the purchase/process!