Good idea? It just happens to be an area I know about. From what I understand the "health" niche is popular.
Yep, that's a great niche to get into, the main reason being that people in that niche have "pressing" problems that they need solved right away. And you can be the one to deliver them help and solutions. Once you build a following you can monetize with related affiliate products from Clickbank, and also Adsense. Good luck!
Blogging about a topic familiar to you or in which you are very interested is always the way to go -- there's nothing worse than running out of ideas and having to force yourself to blog. You can promote digital self-help products, as well as books and other items on Amazon. Just make sure to use a plugin to present an opt-in box when people land on your page -- you want to capture that traffic as much as possible, so you can market directly to your readers in the future. Good luck!