Keep in mind rent control is 1 thing, its a laws for existing tenants but some city's like Newark are so strict that you must get approval for what rent can be even after a tenant moves out and new ones move in. 

They MAY allow you to add another meter on an existing line to the house but may also require a new line to be run.

I'd tell them to return it and buy one that uses the existing connection.

Requires some pre-planning if the ceiling height exists. 

OTA business doesn't exist without the hosts and screwing hosts will be the last thing they would do.

Opportunities still exist here.

Only a couple exceptions exist to their caveat emptor doctrine. 

Are you aware of city of San Diego rules regarding converting existing legal space to ADUs?  

It will co-exist with other "new" payment methods such as google pay, apple pay and credit card etc, because the target customers are different.

He's seen me grow to four units (3 properties) and I'm thinking, there's gotta be a guide or book that would help him just follow step-by-step how to rent out his existing property and buy a new one.