Probably a good idea to figure out the games you want to support initially, your user base in terms of numbers and then google those particular games and look for info on how the communities suggest to build up your server (hardware, network capabilities to support our user base etc). It's hard to really suggest anything - if I were in your position I'd do the research on the individual games first, figure out the scaling that I'd need for CPU, RAM and storage, just how beefy of a GPU I'd need to provide the experience I'd want, and then sit down and think about available systems.

If it's just you and a few mates, you can be a little more conservative with how you spec this thing out, and could potentially get away with a workstation grade machine vs buying a tower or rack server that will enable you to scale up to a far greater degree (which might end up being overkill for what you want to achieve). TLDR: get your software specs first from the various communities around those games, figure out a baseline based on the most demanding games specs and your user base numbers (how many vCPU or threads do you need? how much RAM per player? how fast do these need to be to provide a smooth experience? how much storage are you going to need? what kind of storage is recommended? what are the GPU requirements? what kind of network throughput will I need? are there any potential issues where inadequate security on the server when sharing it out on the internet, might make not only mine but my friends networks vulnerable? etc). Sorry for the rant but I hope that made sense.