I think you are way too early for a business plan. You are worrying too much and trying to plan too much. If you are still in the early garage stage, you need to flesh things out a lot more before you could even begin to know all of the details necessary for a business plan.
In my opinion, a business plan in fairly unimportant in the early stages. Write it when you need funding or a bank loan. Business plans in early stages will kill your creativity and lock you into a way of thinking. If I had written one, I never would have endeavored into some of my most profitable ventures.
At this stage, you need to worry about 90-day cashflow projections and you need to redo it every 30 days roughly, unless something causes a drastic change. When you start shipping or selling, you need to know what your fixed costs are and where your breakeven point is. After that, the next layer to add on is a projection of what you need to ship or sell to hit a comfortable profitability number. That may be two stages: first stage is sustainability and second stage is the point where all the founders can leave their jobs.
All of this is certainly part of a business plan, but if you flesh it out right now it will be nothing more than a guesstimate I promise you. The information is still coming and still being formed as you go. Do not be bogged down in "paralysis by analysis" and do not let setbacks keep you down. They will come. I try to always keep in mind the old saying, "If you waited for all the lights to turn green before you left, you would never leave the driveway."
If you feel compelled to write something right now, how about a Mission Statement? :)