For your CPU I recommend Ryzen 5700G. Powerful enough for everything you want to do, the TDP is only 65 watts so it’s not going to destroy your power bill, has a decent integrated GPU, and costs only about $200. Another positive is that it uses DDR 4 so you can load up on that for pretty cheap too.
Are you absolutely sure that NPM has an IP from the subnet 172.22.0.0/24? Is there any way you can remove the trusted_proxies setting from homeassistant and then check if it will accept the connection from NPM?
As far as motherboards go, you would probably be fine with any consumer desktop brand but you should probably look for something with dual NIC. If you want something a bit more robust AsRock Rack has some really great options. I’ve been using the X470D4U for about 4 years now without any issues.