My backups are tiered. Some stuff gets no backup at all, some gets even more than 3.And I tend to reuse HDDs that got replaced in my main machine due to size for my backups. Power consumption hardly matters when it only runs for a few minutes a day.

It is impossible to fully eliminate the risk but with a decent backup system in place it is somewhat unlikely to lose all of your data.

The 321rule should be used as a baseline. Your local backup should be snapshotted and somewhat hardened against ransomware (pull backups instead of pushing them, do not mount the backup volume to other machines). Cold backups also help.

Can I construct scenarios in which I lose all my stuff? Sure. But in those, we are either in deep shit anyway (CME, some big astroid) or it is pretty unlikely (targeted hacking)

These old machines make less and less sense. They consume 1-200W at idle but have less power than a single, modern 6 or even 4 core CPU. So unless you need a ton of RAM or PCIe they are really hard to justify unless your power is dirt cheap.

£5 x 40 = 200. £3 x 40 = 120. Seems like you calculated with 20 and not 40. This helps to bring the cost down but even for 250 you can at least a single decent NUC.

Not only a ton of work but likely also much more expensive than you might think even with free boxes. A Gbit USB adapter costs lie $10, you might also need a cable, a switch, maybe (some) PSUs, a power strip, maybe some box to put everything into… And now you suddenly are spending $400-800 on this project just to get a rather lame cluster. Those cores are rater slow, 2GB of RAM is not that much, and 8GB of storage means you also need some kind of NAS.

If you like the project go for it but do not expect this to be a wise way to spend your time and money. 3-4 NUCs are likely less expensive and much more fun.