The most important thing to consider when writing a plan is to take your time, be thorough, be accurate, and above all, believe in yourself and your product. Don't just rely on a sample plan, create a proposal that you're proud of, and that you are convinced will compel others to help you realize your dream.
The vendor of the available business will need to see this in order to ascertain whether you are right to buy. It will demonstrate whether you have the correct business acumen required and whether you are serious about the business. It will also be useful when it comes to the financial side of things, but more on this in a minute.