I am just repeating a commonly used phrase with that tbh.
I would Imagine it comes from the fact that a backup also safeguards agains other failures by creating more distance between the Copies. If you are hit by a ransomware attack Raid wont be much good. If your PSU sufers a failure, it might cause an overcurrent/voltage in both hdds. If you accidentally delete something it will be deleted on both drives, etc.
If you go that route I’d want to make sure the cpu is at least somwewhat recent.
My first server was an old office PC I bought used as well, but I had real problems with it, because the CPU was lacking some X86 instructions which is why I could not run a specific service I wanted to as it used those. (And if you want to run jellyfin in the near future you should make sure to buuy a cpu that can also do some hardware encoding/decoding as doing that in software on a low end CPU can make the experience somewhat sluggish.
I don’t think ssds would be viable with a total budget of 500€ and the need for >=8Tb of storage.
No need for virtualization (so no Proxmox, TrueNAS, or Unraid) Run lightweight containers for web services like Immich, Paperless-ngx, Pi-hole, and custom services I’ve developed
Do you not consider containers virtulizations? Or do you plan to run your webservices in the same context? Because I would really suggest against doing the latter.
However, I’m unsure if I really need RAID since I’ll perform regular backups.
Raid is not a backup anyhow ;).
If you don’t need the capacity or redundancy of raid I would suggest you buy a single >=8Tb drive. It should be a bit cheaper (For example, I recently payed ~270€ for 16 Tb, which I’d reccomend over paying 240€ for 8Tb in total. There are probably also 8/12 Tb drives for less then 240€.). It will also use half the power as that does not really scale with capacity.
Edit: there are a bunch of 8tb drives at 180€ here for example.
I googled around a bit and it seems the main concern is being seperate sowftware-wise, not mainly location wise (although thats a nice side effect). So mainly avoiding fun stuff like https://m.slashdot.org/story/112253.