Wifi can easily do 20m with decent equipment. With a directional antenna can go a surprising distance, ie many KM, if there isn't a physical barrier.
I have some unifi APs and my garage openers keep connecting to the AP at the opposite side of my house about 25m away through a few walls (including one cinder block wall). I have one unifi AP in wireless bridge mode that goes 20m-25m (what the openers are supposed to be connecting to), which has its own IP that could be pinged.
For non-ethernet/wifi solutions, there are loRaWAN based power meters/monitors/switches, but you may have to diy one of you are looking to keep costs down.
The main benefits to paying for certs are
The only thing that matters to most people is that they don't get cert errors going to/using a web site, or installing software. Any CA that is in the browsers, OS and various language trust stores is the same to that effect.
The rules for inclusion in the browsers trust stores are strict (many of the Linux distros and language trust stores just use the Mozilla cert set), which is where the trust comes from.
Which CA provider you choose doesn't change your potential attack surface. The question on attack surface seems like it might come from lacking understanding of how certs and signing work.
A cert has 2 parts public cert and private key, CAs sign your sites public cert with their private key, they never have or need your private key. Public certs can be used to verify something was signed by the private key. Public certs can be used to encrypt data such that only the private key can decrypt it.