https://adult-wordpress-theme.com/
I recommend to get a professional looking responsive theme which may cost a bit money but it definitely pays off. Now, Google is de-valuing unresponsive site design in an attempt to further integrate the web into the mobile experience.
With this theme comes an entire pack of all the necessary components for a successful website. It is responsive and retina ready, full of awesome layouts and styles. Not to mention a fast and ready to help support.
Hosting your own videos will mean you will need a lot of space and bandwidth on your server so better to embed videos. The mentioned theme is perfect for embed videos.
Choose whichever niches you prefer - better to focus on something you know and love, rather than making a mishmash of everything; especially as you are just starting out.