https://docs.opnsense.org/manual/hardware.html

Depends on what features you want to enable and how many devices are using the network but a relatively recent quad core above 2ghz is probably plenty. If you want to do packet inspection or such, more Is probably better but you're not going to need a 32 core or anything. Is there a specific reason you want a 5gb local network, or just want the fastest network you can afford?

No expert here, but you should check on Protectli, they have good stuff, small form factor(mostly), fanless, serial com which is convenient in some situation.
I have one (intel not amd) that is currently not in use for personal reason but when it was running, I found the hardware very reliable, silent and not hot.
Prices are not as good as some cheap hardware you can find on Aliexpress or Amazon but somehow I trust more company like Protectli than any noname because at least with Protectli devices it is possible to install coreboot to override the default bios. (BTW when people check the bios on their cheap hardware then things are getting weird, which version it is ..., doesn't sound like something I would trust to manage a network, just saying).