I just pulled out my old server from before the move a couple years ago. It's an optiplex 7010. That's hardly a powerhouse, but it'll be enough to run nextcloud. I also found a WD 4TB HDD.
So, time for some upgrades.
I ordered a second identical HDD so I can have RAID storage. I also ordered a 250GB SATA SSD for the operation system, and a caddy to hold it in a spare disk bay.
Then a 32GB ram kit, the max supported.
Total spend: approx $100 after some credit card rebates