When asked what to focus on when starting ads with a new client:
The first thing I do when starting a new campaign is getting a breakdown from the client about how their average purchasers got there. What was the organic funnel that has already been working for them?
Then, I try to recreate the process within ads.
As far as targeting I start with what they have. Who are the people already in their funnel who have not bought? (Website visitors, email subscribers, FB likes, etc).
These are my best potential buyers. If I can learn what type of ads cause them to buy, only then I find larger audiences and walk them through the same process.