You don't need a dedicated PC as a NAS (network attached storage), you could do it cheaply and simply by plugging in an external USB drive into your router since most modern routers support USB hard drives. Your router is on 24/7 and would use the CPU/RAM in the router itself to map and deliver the files over your network. It would work much in the same way as a FreeNAS or unRaid server if the intent is only a centralized location for files. The next step up would be an external HD with built in NAS capability. This would plug into your network and would be a simple low power device like a Western Digital MyCloud. The nice thing about this is you could run applications in addition to just sharing data, like running a Plex server or automated backups. Arguably the next step up would be a multi-bay dedicated NAS solution like a Synology or QNAP device. They use similar PC hardware components (Atom/i3/i5/i7/Ryzen) on a dedicated custom NAS software, with available plug-ins that allow you do more. Lastly would be custom implementations using off the shelf parts and software to do the same purpose, but with the added flexibility and omplication of building it yourself. We can leave SAN's out of this discussion.