Only if those all look rosy should you start. And if you want to start that way, you need to do surveys of other businesses in the area as well as your potential customers. This is what people learn in business school, and starting without having a business school person on your team is a great risk.
Also, funding is not your problem. This type of business you are talking about is what bank loans are designed for. The bank loan is made out to the company, so if you fold, you are not personally liable.
If I were you, I'd only do this if I were really passionate about milk and cookies. Personally, I like neither milk nor cookies, so you would not have me as a customer.
At the same time, you cannot start such a store without focusing to a large part on cofee. Coffee is addictive, and will make your customers return. Milk is not addictive, and it tastes the same everywhere, and is much cheaper. What incentive do you have towards making people return to your store when the milk is just the same as everywhere else, but overpriced?