[inapis]

Buy commodity computer parts and build a PC. If it's strictly storage, then a cheap pentium/i3/Ryzen 3 would suffice. If you want to do more, then spec for a higher CPU. PCPartPicker should be able to give you the list of compatible parts.

Then depending on what you want out of the device, do one of the following -

1. If it's strictly storage and you want the niceties of ZFS, install TrueNAS 2. If you want expandable storage without ZFS + VMs + Docker with a nice GUI and are willing to pay for a license, install Unraid. 3. If you want ZFS-backed storage + VMs and no docker, and maybe some datacenter-style capabilities, install Proxmox 4. If you want all of the above and are willing to manually configure everything, then install Ubuntu Server + Cockpit for managing the machine headless + Portainer for managing a large number of docker containers.