Cheap rack monitors or KVM IP

Hi everyone! I have a couple of old desktops as servers in my rack. Sometimes , very rarely, I need to use the machines console.

Slide-out rack monitors are incredibly expensive here in Germany, so these are not an option. Next I thought about USB monitors - don these work out of the box with Linux or do the need special drivers? Or would I be better off using KVM over IP? Do you know of other not-so-expensive solutions?