Consider buying dedicated firewall hardware - check out “DEC677 – OPNsense Desktop Security Appliance”. I don’t have experience with opnsense hardware but I use similar for a different os. The idea is that you won’t run into compatibility issues since opnsense will test all updates on their own hardware before releasing them and should be optimised, etc.

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?