Like other business plans, the contents of an automobile dealership business plan cover four primary steps towards success. These steps are "opportunity", "capacity", "activities", and "results".
The industry, customer, and competitive analysis in your business plan proves the opportunity. This should identify that customers in your local market are in need of a dealer of the type you've chosen to be because competitors are not fully serving their needs, or because their demographics are changing (more population, richer, poorer, more families, etc.) The opportunity could also be that a certain combination of services by your dealership could improve on what competitors are currently doing. The overall industry situation should look ahead to how the car sales market will be doing six months, one year, or more in the future, when the dealership will actually be open.
The experience of the management team, your license as a car dealer, and the financial resources the partners can bring to the table cover the capacity of the entrepreneur to jump on this opportunity. Funders reading the business plan expect that there will also be an element of cash missing from the entrepreneur's capacity at the moment. Otherwise, they would not be reading the plan in the first place.
Marketing and operations plans cover the activities the company will engage in to bring in car buyers (and car sellers if you are a used car dealer as well), and run the business as efficiently as possible. As a small car dealer, consider how you can make up for this lack of scale, and the cost savings that come with it for dealer chains, through creativity and hard work.
Finally, the projections of business success and financial section of the plan show the results that you and investors should expect. If results take into account the full costs of the methods you've described, and include adequate compensation for staff and key team players, readers will be less likely to question elements on the cost side. If your revenue projections are based on reasonable, researched assumptions about the car buying behavior of your local market, then readers will be more likely to accept this side of the projections as well.