We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
10TB for €120? Are these used drives?
I would strongly consider bigger drives. 18TB or more. Using small drives is expensive in the long run. More W per TB, more bays per TB, need for bigger and more expensive cases, need for HBAs and maybe some high end platform to get enough PCIe lanes etc.