Sending Email to a Single Contact

I'm a real estate agent and want to save HTML email templates for different portions of the home buying process. Each time a client hits a milestone, I'd edit the details for their sale and send to a single (or maybe two) email contacts. Is there a way to do this without having to create a new segment within a list each time for their name? Is there a way to attach documents too?

I saw threads from several years ago that said they added this function but I don't see where it is.