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.

If CPU speed is not particularly important, I suggest getting some Wyse 5060 thin clients. You can get these for around $30 per unit on eBay.

Then upgrade the storage and RAM. They can take up to 2x 8GB DDR3 SODIMMs. Storage is a SATA DOM. You can just remove the board from a lot of 2.5" SSDs and use this. I’ve done this with a 500GB Samsung EVO 850 and it was pretty easy.