I don't think it's too early to apply for YC. We hadn't built anything when we applied (although we had spent a few weeks brainstorming ideas). We built our prototype between when we were accepted for interviews and when we actually interviewed. Even if you don't apply, filling out the YC application is a great way to think clearly about your idea.

YC accepts good people who's ideas they don't like and then try to convince you to do something else. We fell into this category. I wouldn't spend too much time writing a business plan though. Start building things and experimenting.