I have an old PC that I have been running as a server for around 5 years now and it has been a really fun project to work on in my free time.

To answer your specific questions:

• No, you generally would not need a gpu (other than initially getting things set up if your cpu doesn't have integrated graphics)
• You don't need a specific CPU, but if you are planning on running multiple servers simulatenoesly, then a higher core count would certainly be helpful
• The size of storage you need will be super dependent on which servers you want to run, so it would be a good idea to look up the server install sizes for the games you want to run. That being said, it is generally pretty easy to add more drives later if needed

For some general things to think about:

• You may want to look into running a hypervisor (an OS designed to host other OSes in containers or virtual machines). If you are going to be running a number of servers, having them hosted separately on a hypervisor can make it a lot easier to take some up/down without impacting the others. Personally, I run proxmox as my hypervisor, but there are a bunch of options out there
• It is also important to consider the energy costs of running a server. It might surprise you how quickly an atound the clock server can eat up power, so just keep in mind that you might see some changes on power bills

Hope this helps! It's a fun hobby, and I hope you enjoy it.