Debian or Ubuntu Server (or something specific to servers purpose, like OMV, etc).
… but ProxMox (a hypervisor, Debian based) doesn’t have much overhead & runs on old PCs pretty well. And with that, you can pretty much try any distro (as a full virtual machines, perhaps with dockers within it, or as a lightweight containers that are really resource efficient). Or separate containers for each purpose (for beginners, there are like TurnKey solutions to stuff like NAS, it takes literally a few minutes to set up).
Backups (snapshots) are easy too, and a later migration to a better/next server is basically two clicks away.