Debian is the OS.

Proxmox builds on Debian to turn it into a hypervisor. That’s an OS for managing VMs and containers. Proxmox (like most hypervisors) gives you a handy webinterface.

There are easier hypervisors to use, but Proxmox is great for learning, and allows you to expand well later.

tbh, except for gaming and AI, you don’t need nearly as much processing power as you might think. I run most of my stuff on a (fairly new, can’t remember which one exactly) i5, and it sits at an average 2-3% CPU usage, with the maximum its recorded being 10.9% (there’s two VMs and 8 containers running on it). 11th gen i5 will probably be more than enough to start with.

In my non-professional opinion, what you really want to look at is how much power your system is going to use when idle, since that’s what (small) servers are doing most of the time. This is not something specified by manufacturers, and it depends on all the parts going into the computer (CPU, mainboard, PSU, fans, how the BIOS is set up, …).

I would also recommend getting at least 16 GB of RAM (honestly I would say 32, if not for the current prices). Containers don’t use too much, but if you want to set up VMs for services, each VM will want at least 2-4 GB for itself.

LXC-Containers (en.wikipedia.org/wiki/LXC) are like extremely lightweight and slightly less secure Linux VMs. They are similar to Docker, but while Docker is containerizing applications, LXC containerizes Linux distros.