A used Sophos XG 115. Has Intel Ethernet interfaces which is preferable for BSD compatibility. 8W idle. I power it off a 802.3af to 12V splitter.

If you want maximum speed a Lenovo Thinkcentre m720q has a desktop Intel CPU and a PCIe slot. You can add a 2x SFP+ NIC and PCIe riser to get 10G.