When I started by business off of a franchise model, I had the same questions. At the very least I would make sure you have a general liability policy. There are probably not a lot of risks involved with your business, but if somebody trips and falls getting in or out of your air conditioned van and gets injured, they may expect you to pay their medical bills. It's good to be covered for this.
You can shop around for a good rate from any number of reputable insurance companies. Just search for "business liability insurance" in your city. Above all, talk to an attorney. It will be worth the time and money in the long run. If you know your business idea is validated and will work, if you are going to the lengths of creating a business plan and marketing plan, then you should be talking to an attorney. At the very most you'll probably be paying for an hour or two of their time at the most.