A NAS is any computer with space/connectors for drives and an ethernet port… besides the drives, it doesn’t need to be powerful or state-of-the-art, and there’s really no reason it should be expensive.

Of course companies will be more than happy to sell you an outdated J4125-based computer with 4 disk bays for over 500EUR, but that doesn’t mean you have to bite.

As for RAID, if you want to use it, just setup mirrored drives (ZFS, BTRFS or even LVM) and be done with it: you’ll need backups anyway so don’t overthink it. Unless you want to avoid downtime (which isn’t probably a big issue for most of your data?), you can do without RAID and just restore from backup if a drive happens to break.

If you don’t want to build your own PC, I’ve heard good things about these: aoostar.com/collections/nas-series (beware: I didn’t try any of them - my N3150-based NAS is not old enough to need replacement yet)