Wanting to replace my old, ancient PC that I’m using as my firewall with a small/micro form factor device. I run three zones (red, blue, green) so that my wireless is separate from my wired zones. So it would need three NIC ports.
If you want cheap power, I got one of these from Amazon
Two Gbe, Two M.2, (three if you drop 802.11ac WiFi).
Memory expandable, Intel N150 CPU.
Built in offline mains rated PSU, which is really good feature.
Two HDMI, Four USB, (two at USB3.2). Really tiny box, 120x1220x40mm.
Wifi, Bluetooth. If you really do need three or more ports, use a web managed switch and VLANs.
Trigkey-N. Total bargian IMO at £160.00 (ukp)
I use a mini PC that has 2 nics for red and green. I use USB nic adapter for blue.
I just bought a Zima board to set up. It comes with 2 nics. It has a pci-e slot for external use. I have a dual nic card to connect. The mini PC is overkill and the Zima might be a good replacement.