Basically, you can use any old computer, so maybe you have one laying around somewhere. This is ideal if you are not sure yet about the server idea. The hardware we have is more than enough for the things we want to do, but do not invest if you are not sure
Another thing to realize is that you will not be working with fancy graphical interfaces, but that you will have to go down all the way to the command prompt. So a little bit of knowledge of a linux operating system is a must. In the end the sever is standing somewhere out of sight, silently humming with only a power and internet chord.
With our first try, we had the graphical interface installed, and were surprised we did not get the server working when the monitor was removed. We fixed this with a stub and teamviewer. But if you notice that you are only working on the command prompt, then the useless overhead becomes abundantly clear.
So choose a linux version and install it in the minimal installation, headless, with no need of a monitor. Sure, you will need one to do the installation, but after that you won’t need it anymore. We chose CentOS as an operating system, and are very content with it. In the end it does not matter, really, and linux is both reliable and for free.
The easiest way to install is to make a bootable usb stick, and then power up the computer (with monitor, keyboard and mouse) with the bios set to boot from USB. When all goes well, and the installation is finished and the computer rebooted, all you will see is a black terminal screen.
This is a link we used : https://www.howtoforge.com/tutorial/centos-7-minimal-server/