The best place to start is usually eBay or a functional equivalent. Businesses dispose of the prior-generation tech more or less constantly.

Another poster suggested buying a server. While it may be a good idea from the standpoint of learning, I would like you to keep in mind that servers can be noisy, so adjust that recommendation to your living circumstances. An alternative is to buy a PC workstation or even a high(er)-end office PC (for example, Dell Optiplex, or HP Pro Desk / Elite Desk, or Lenovo ThinkCentre / ThinkStation). Those are designed for corporate use and have multiple options for upgrading and expansion (empty slots for RAM, SATA drives, PCI cards, etc.). You can start as low and as far back as i3-6xxx, but obviously, the more recent and more muscular, the better. See what you can get for your money in your home market. Get some extra RAM (16 GB should be enough for starters) and an SSD to install the OS on. Speaking of the OS, start by installing Proxmox (it's a Debian-based hypervisor, meaning, an OS designed to run other OSs on top of itself) and run the rest, whatever it ends up being, as virtual machines.

And yes, you can run your NAS as a virtual machine, too...