>>106244970
I say get a M920q lenovo with the PCIe card adapter plus a x710-da2 or da4. Lots of examples out there for opnsense running this perfectly. With a DA4 it might get a little toasty, but it seems to be fine for the thousands of people doing it.

I prefer the HP Elitedesk SFF if you have the space. More breathing room for the NICS if you get the low profile variants of hte x710. Idles even lower than the m920q on Win10 but Opnsense has shit C state kernel coding so its probably going to be 10w-12w floor power costs with either.