If a document needs to be developed that requires input from other disciplines_Finance, HR, Property & Facilities, Marketing, Procurement/Supply Chain_ then most likely you are looking at a team building effort to get the job done. In any event, don't look at the task as only as a roadmap that leads to a profitable product or enterprise. Business plans are a great way to build team buy_in, force a thorough review of options, define objectives, establish benchmarks to judge performance, and help arrive at a plan_of_action. Ultimately, it can lead to a Project Management approach to implementing a plan and that can be as involved and detailed as is necessary.
I have written business plans for all manner of industries: a coin operated jukebox company, airlines, travel companies, new product launches, and anti_aging product companies. It is not necessary to have a passion for the product or the company to write or develop a business plan. What you must have is a passion for aggregating information, getting involved with and understanding the service or product, and understanding the financials of the product or service. By financials I am not referring to having a CPA before you undertake the task, but rather understanding the presentation of the information and analysis/ numbers to support the activity being planned. Financials are important because they are the score card in the world of commerce.