Does the SBC need to be x86? If not the way to go is Pi3 or 4.

If you need x86, I can recommend any of the Lenovo/Dell/HP Mini systems (granted they would likely be overkill if it’s just k8s)

Is this for learning k8s or self hosting?

For learning I have 6 Pi 3Bs that I use for testing various things, it’s pretty nice, light weight easy to bring to conferences if I want to.