If you have the space, any old computer. If you like it compact, you get a lot of bang for your buck with the small 1-liter PCs. I like the Lenovo m920x in particular: 2 x m.2 plus a 2.5 SSD.

For the OS: Proxmox. That will let you play around with VMs and help you learn how to run stuff without having to reinstall the entire computer when something goes wrong.

When you feel like you've outgrown that, a next step could be a dedicated NAS.