If you are running a media server, depends what you going to do with it. Storing media without transcoding you don't need much, ex-gov computer from last 10 years x86_64 CPU that could store a HDD, you could use openmediavault (Linux), Unraid (paid - linux) or freenas (freeBSD) in a JBOD config and a ssd for cache (so you can serve databases, and metadata quickly, like plex or Jellyfin) and it will be much faster than MyBook Cloud.
Transcoding media, that is converting media from a format to another format so you can be compatible is a different story. You will need at bare minimum intel with quicksync or a fast CPU for software encoding.
Generally with DIY NAS software, you load a docker, point the docker directory (fake) to your jbod directory and it will just deploy on the webUI port you assigned it too.
Generally I will do this imo. Cheap intel PC (eg. Ex-gov) with a HDD > upgrade to SSD for a cache drive > DIY PC with plenty of SATA ports > upgrade to NVME > Cheap GPU for transcoding OR SAS card for more harddrives.
If the data is really important, make sure you have a parity drive and a backup solution.
I absolutely agree. Unraid is well worth the price.