Been there,

I started with a discarded desktop for my server, if you have choices you should probably focus on the fastest cpu clock speed. Cores don’t matter for Minecraft, any non broken minimum gpu will do as it doesn’t need to run the gameclient itself, Memory is the most easy part to obtain more of later. For just a mc server 4GB should do fine.

It be better to put a linux distro on it, currently my server is in a docker on Ubuntu. If something happens and it needs to restart the pc and docker can boot without even needing a monitor to login. It knows what to do and is very stable and reliable.

Of course if your like me at the start linux will still scare you. Go ahead and install the latest windows from the creation media thing from Microsoft. Provided you don’t care about personalization on the desktop you don’t need to pay to legally use windows. It wont be as reliable at times but otherwise work fine.

Big warning though, At some point you might end up with a custom build home theater server pc running with headless linux. not just minecraft but also numerous other self hosted things like cloud storage, media player,… I have a setup where i can insert old dvd into the server and it automatically rips and stores it on the server and i can just watch it on any pc or phone. Great if you have kids and a pile of old dvd in a local language that is hard to find online.

I cant tell you exactly how i got there since the days of running minecraft on a windows server, it just kinda happend organically.