Don't use a synchronized folder as a backup solution (delete a file by mistake on your local replica -> the deletion gets replicated to the server -> you lose both copies).

old pc that has 2x 80gb, 120gb, 320gb, and 500gb hdd

You can make a JBOD array out of that using LVM (add all disks as PVs, create a single VG on top of that, create a single LV on top of that VG, create a filesystem on top of that LV, format it as ext4 filesystem, mount this filesystem somewhere, access it over SFTP or another file transfer protocol).

But if the disks are old, I wouldn't trust them as reliable backup storage. You can use them to store data that will be backed up somewhere else. Or as an expendable TEMP directory (this is what I do with my old disks).

My advice is get a large disk for this PC, store backups on that. You don't necessarily need RAID (RAID is a high availability mechanism, not a backup). Setup backup software on this old PC to pull automatic daily backups from your server (and possibly other devices/desktops... personally I don't bother with that. Anything that is not on the server is expendable). I use rsnapshot for that, simple config file, basic deduplication, simple filesystem-backed backups so I can access the files without any special software, gets the job done. There are a few threads here about backup software recommendations:

• What backup service do you use? - Lemmy.world
• What are your backup solutions? - Lemmy.world
• How do you guys back up your server? - Lemmy.world
• How do you backup things to your server? - Lemmy.world

In addition I make regular, manual, offsite copies of the backup server's backups/ directory to removable media (stash the drive somewhere where a disaster that destroys the backup server will not also destroy the offsite backup drive).

Prefer pull-based backup strategies, where hosts being backed up do not have write access to the backup server (else a compromised host could alter previous backups).

Monitor correct execution of backups (my simple solution to that, is to have cron create/update a state file after correct execution, and have the netdata agent check the date of last modification of this file. If it has not been modified in the last 24-25hrs, something is wrong and I get an alert).