From your description, this new box would not be necessarily have to be a full homelab-in-a-box but needs to be enough to run on its own, with possibly an umbilical cord to your normal homelab for regular syncing. The new box needs to be fairly user-friendly, in the sense that someone else can connect it to their monitor/keyboard/mouse, enter a password, and be able to browse all the documents.
The first thing that comes to mind for me is a NUC or other small form-factor PC, with capacity for your desired SSDs. On a daily basis, this would sit somewhere convenient, like a home or maybe off-site from your homelab, with only power and a network connection. But it would run an OS with a GUI – GNOME? – even though it mostly runs headless. All your syncing could be done with rsync or whatever, and neither your homelab nor this machine should require the other in order to function properly, retaining independence. This machine could then be easily disconnected and tested semi-annually to make sure that it will work properly when the time comes.