What I want to do:
Send out HTML emails to a variety of to-be and current clients, depending on many factors. Sometimes, I have a large amount of clients to send an email blast to, other times it is only to an individual. 
These email addresses can come from emails, the website via Sales IQ, or completely out of the system from Sign ups or contact page. 

The problem is that I seem to be missing the best way to do this. I would like to somehow only input my clients/leads/contacts into one product, one time and be able to choose when and who I send a campaign to without having to keep creating a LIST for each time for even a single email. (I am doing this through campaigns because in Mail, I can not use HTML messages. This also frustrates me to no end.)

I have watched numerous videos, but they are all too general and just show me features. I really need to figure out the workflow basics, as they relate to basic online B2B startup. Can anyone give me some advice? Maybe I am completely missing something basic so I welcome any input.
TIA