Seconding the other user’s post, it’s just storage. You can use it temporarily for the Plex server but I highly recommend offloading that task to a real computer.
A used Optiplex is usually the go-to because it’s low power and capable of doing the job. Just make sure it has enough RAM to handle the containers you wanna put on it.
Docker is a little bit of learning but you’ll get the hang of it. I found docker-compose to be much easier to learn. Portainer or Dockge can help by giving you a GUI, if that’s your preferred style (like me).
I don’t really have a tutorial suggestion but this is a great resource if you haven’t seen it yet: wiki.servarr.com
I would also recommend using dockge to help create and manage your stacks.
When you’re setting up the services, go through each section and take note of what you don’t understand. The services tie into each other so it may be beneficial to set up your stack first (docker compose YAML text file, managed by dockge) and configure stuff together.