Proxmox is both easy and powerful so it’s a great choice for self-hosting. Your specs are fine, you’ll run out of disk space first if you want a media center and then you’ll run out of RAM way before your CPU can’t keep up with the work.
I recommend Jellyfin over Plex as well, as a small note.
How many SATA headers do you have on the mobo? If you just move that into an enclosure that can handle a couple of HDDs you’re probably golden. And I bet that Mobo can handle 32 GBs of RAM as well, if not 64? So you have a clear upgrade path before needing additional expensive hardware.
Overall I think it’ll be great! How’s your current network setup? Self-hosting stuff puts some demand on router and wiring that you don’t really have if you don’t selfhost anything.