Cheap fanless motherboard with 2 NIC's: https://www.amazon.com/gp/product/B006ICQ3FK/ref=oh_aui_deta... ... pay attention to the power supply brick, you might need a little adapter dongle for the mobo.

and a cheap case: https://www.amazon.com/gp/product/B003DXI288/ref=oh_aui_deta...

No hard drive, just a usb stick to boot the OS off and save his settings. PFsense in my case, a BSD variant.