If your tank became septic, take it to a mechanic, probably just needs an oil change. Then you can shit in it and fire said shit to whoever it concerns, sincerely, and with love.
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.