Use an old PC, or build a NAS?

Looking at getting a NAS for Plex and just generic data backup. I’ve been bouncing between buying a NAS, building a standalone computer, using free smaller footprint computer (not sure of the hardware specs yet), and building one using a case and M-ATX board. I have a Synology right now, but it’s an older one and can’t transcode. So, that would continue to store my media, while either computer would do my server hosting.

I do like the idea of having it in one enclosure and being done, but also don’t really care if it’s not.

What would you guys do? I’m not looking for a massive setup, but 20TB~ would do me fine. Probably a 4 bay setup.