I have an HP T740 running Opnsense and it works just fine. You can pick them up for about $100 USD, and they seem to come mostly with 8GB RAM, and a 64GB SSD. That seems to be more than enough for Opnsense, even running VPN’s.

These are just a tad larger than my Lenovo M910Q Tiny servers, but they have a PCI slot, so you can put a second ethernet port (or 2, or 4) of whatever speed you like in it.

Wow. SNOBOL was one of the coolest, Stange languages.