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.

If you can afford it, the easiest way to start might be to buy a popular NAS. I would recommend Synology. The setup and interface are fairly intuitive, especially if you're technical. Then as part of your workflow, just make sure the completed videos are stored on the NAS as they are uploaded to YouTube.