Unless you have a sure-thing outside investment deal just waiting for your business plan, don't waste your time. If you decide to go ahead anyway, go ahead and "outline the goals of the site" but throw away the long-term vision and financial projections.

I can't think of anything more useless than long-term anything/financial projections for a startup. Both are simply fantasy, with no basis in reality.

I know it's really cliche these days but you need to identify the one, or a maximum of two, goals for the site and just build it (minimum viable product). Trust me on this. I spend 2+ years building something grand that just fell on it's face. I would have liked to have figured that out 2 years earlier. Every "feature" you think is going to make the site great is really just a liability and time sink.

Unless you have a lot of cash, your only asset is time and writing a business plan is wasting it.