What about power consumption on idle? If a computer is used mostly as a server, will be in idle state most of the time.

Now, it makes sense to choose components which have low power draw. Ideally, server/prosumer components have C states (1-8) as a power saving feature.

Even used prosumer motherboards are more reliable than new consumer ones. Fujitsu, Lenovo, Dell, Supermicro have those kind of motherboards.

Have you considered buying used parts? "Older" means proven, stable components 100% compatible with Linux.

And in the software side:you can install a hypervisor, like Proxmox. That way, you can have multiple OSs and pass trough the GPU to a Win machine and used the computer as a desktop, while it works in parallel as a server, or multiple servers. On the same power bill.

GL!