While it may seem more lucrative to choose your niche going by CPC ranking, I wouldn't recommend going that route, especially if you'll be writing the content yourself. In that case, it's best to stick to topics you're excited about covering. In fact, even if you end up outsourcing the writing, you want to be able to make sure all the facts check out before you post the content.
In any case, if you want to keep people coming back, you definitely don't want to focus only on keywords and SEO. Instead, offer them good and interesting content. On the other hand, if you'd simply like to take advantage of SEO and make some money, follow the tips in the first comment.