StudioPress is a WordPress hosting service and framework that is designed to make setting up and running a WordPress site much simpler and easier. StudioPress comes with its own unique themes and SEO tools, collectively known as the “Genesis framework.”. Their affiliate program is solely for referrals to pay for a StudioPress framework account or buying a StudioPress theme. Previously, the affiliate program also included web hosting, but this is now managed separately by StudioPress’s owner, WPEngine.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.