I know some businesses implement relatively complicated PKI structures, but on average will managing PKI in small bus. be a simple task ?

I know software (browsers and other Apps) mostly handle PKI without user intervention but I don't expect a restaurant or local library would use a much more complicated PKI structure beyond that.

Depends on the business and what they host internally. Some will have websites with digital certs from a trusted CA and some will generate self signed certs. Most medium sized business and larger will run some sort of certificate services either on a server or a router (A cisco router can be a CA). If its all hosted externally then the pki will mostly be handled by the hosting solution. These days most sites run certs from a trusted 3rd party CA (Verisign or others) only because how modern browsers act with self signed certificates.

excellent...