Lenovo M600 Tiny. It's nuclear bomb proof, runs on virtually no watts, has no fan and runs headless linux fine. Also supports proper storage like M2 SATA disks and doesn't have the horrible power supply issues you tend to get from Pi's. Usually available relatively cheaply on eBay.

I was running a cluster of 4 of them when I was learning about Kubernetes

Spec sheet: https://psref.lenovo.com/syspool/Sys/PDF/ThinkCentre/ThinkCe...