Depends a bit what you want to do with it. Replace your router? Just apply FW rules in your LAN? Intrusion Prevention/Detection?

Make sure the Network Interfaces are performance under BSD.

Check this if you haven't already: https://docs.opnsense.org/manual/hardware.html

Almost everything works as long as the Ethernet chipset are Intel.

For the wifi is a bit (or a lot) more complicated, you will need to research.

How many ports, ram or cores is something that depends of your use case and you didn't specify

But if you want a small guidance I used a second hand sophos machine for ebay, and without idp/ips is sitting very comfortably on my home network

Don't buy shit from AliExpress.