i would look at pretty much anything 8th gen or newer intel, specific cpu doesnt matter a ton, lower power the better tbh.
JDM’s recent guide for everything but the drives and chassis would be a great Plex server host [Official] U-NAS 8-bay Mini-ITX NAS (OTiS - 8th/9th gen Intel) which is a continuation of the original OTiS builds [Guide] OTiS 1.0 - Build your own Intel QSV HW transcoder!
as for hardware, it wouldnt be very hard to shove the hardware JDM referenced in the U-NAS build into pretty much any standard 1U chassis, such as this cheap istar https://amzn.to/3okslAP. However if you dont mind getting your hands dirty, i highly reccomend checking out the 1U shenanigans channel on discord and this post [Guide] Spice up your rack with these stylish & sexy chassis for your pfSense build!