I have been up and running a used/"refurbished" HP T730 thin client with an Intel I340 nic for almost a month now. working perfectly and routing near gigabit WAN just fine, its an appropriate "everything you need nothing you don't" in a semi small reasonably power efficient package, I have not seen over %30 CPU load. I am running on "bare metal" if you want to virtualize Opnsense you will want the intel I350 nic s the I340 is missing some of those features,. I have disabled the onboard real-tek nic in the bios.

​

fun feature the T730 has a tiny/crappy pizio (SP?) speaker, open sense plays a tune on shutdown and a diffrent one when everything is up and running, handy to know when to login on reboots. in the bios I have it set to auto boot on power loss/return, handy for a router.

​

since you asked for Amazon links: where I got mine,

https://www.amazon.com/gp/product/B07GX7BZQ8?psc=1, I got mine from Evasales101 out of NYC for $169, (look on the right side) came with a keyboard and mouse that I did not need, but did not come with the stand that I could have used, you can also find them on Ebay for a wide range of prices, with different amounts of Ram (max 8GB IIRC) and "drive" space, even down below $100 if you want to add your own M.2 sata SSD and ram.

​

Intel I340 https://www.amazon.com/gp/product/B003A7LKOU?psc=1 you will need the short bracket for the t730, mine came with such, some reviewers said theirs did not, be on the lookout for Chinese fakes, mine was real, the fakes work just like the real thing for a little while and then die. https://www.servethehome.com/investigating-fake-intel-i350-network-adapters/