There Is not much Difference Between I7 and I5 for Power Draw. You can Get one for Around $100, an Absolute Steal. Actually comes With Win 10 Pro, so You can setup Opnsense to USB.

I Recommend a HP Office, Refurbished HP Elite 8300 I5 Quad Core Small Form factor PC. it idles around 33W but its been faithfully doing its thing for 10years.pretty old 3rd gen intel xeon with ecc. All in one NAS/router/other thing solution made the most sense for me since i already had the server. I personally run a dual port i350 in a VM on Truenas Scale. just the price of some 2.5gbe NICs and a switch get you close to the units cost. The chinese celeron based 2.5gbe units are right around your price range. By the time you buy the device and a 2nd NIC youll be around $150-200 on the m720q.

You can do funny stuff with VLANs and a managed switch to make a single port work up to 500mbps. Most compact with lots of CPU would be a mini PC from dell, lenovo, hp with a 2nd NIC added. I have HP 290 with a i3-8100 which is similar to some HP prodesk models that draws 13W idle with a dual port i350. Cheapest option is going to be a hp, dell, lenovo used business SFF pc with built in intel NIC.

Reddit opnsense