I'm fortunate enough to have not only a friend who writes business plans, but another friend who was a commercial lender. Thus I have friends on both the author and audience side of business plans. I myself am no expert on business plans, but these friends have given me valuable insights into what they must include to be effective.
My retired commercial lending friend recently suggested that one approach towards pitching a product to investors is to put together a simple, straw man business plan before spending too much time and effort fleshing it out. The idea is to put forth the basic value proposition and projections and use it to get feedback on how to refine it.
Of course, you will need to do some market research to end up with a compelling and credible plan. To some extent, the nature of this market research may depend on feedback you get from presenting your straw man business plan, however. So how do the business plan and the market research fit in with each other, given this cart-before-the-horse problem?
The answer is to iterate on both your market research and your business plan. Compose your business plan at the same time you hire a skilled product manager to research the market. Write the skeleton business plan, fleshing it out iteratively as the research findings and data collect. Start with formulas and metrics and plug guestimates into them, then put "real" numbers into it as data accumulates from the research.