An old PC is the best NAS, even if you choose a dedicated NAS now it’s likely that you’ll want to “upgrade” to an old PC in a year or two. Unlike premade NAS boxes you have full control over the software and can modify the hardware as needed. You can undervolt/underclock to save power too, so the main difference is only the physical space it takes. Having the ability to run docker containers and VMs on the same device is incredibly useful, and you’ll get significantly faster transfers despite the drastically lower cost.