Check out the TinyMiniMicro series from servethehome
- Do not buy something that uses ARM. Things like TrurNAS or Proxmox simply don't run there. You'll also run into problems with Docker images being available only for x86
- don't buy a Raspberry-like computer. Yes, they draw low power, but for the same price you could get a TinyMiniMicro-esque device which is upgradeable (e.g. add a 10gbit NIC) and an order of magnitude faster