So linux PC #3 at my desk is going to become a server of sorts. Backup server for borg is one of the key roles it can do. Another one is Virtualbox server or PC running vboxes natively, depending on how I set it up. Another way of working around disk space limitations on MainPC without spending a lot (or virtually any) money.
I have a pile of old disks lying around and there should be enough WD Blacks or REs to give it a 1 TB RAID-1 array with mdadm without it costing me anything. However it has only 8 GB of RAM which isn’t really enough for its new role. The memory from the other computers isn’t likely to be compatible with it and memory is expensive (this motherboard only has two memory slots which is an unusual choice for me, but the PC was expected to have a low usage when I built it and cost was important). I would have to decide whether to put 8 GB or 16 GB extra into it, but 8 is much more likely, but too much to pay for at present.
It will run Debian, but a desktop version with GUI for the present time. For borg it will be able to act as a server to back up the other two PCs on this desk which have all the stuff stored on them (which adds up to a lot of stuff) and the backup disks can be optimised by having stuff from both PCs stored on them.