We don’t intend to charge money for our product at first – should I still talk about our business model?

Yes! Investors need to know that they have the potential of making my their investment many times over. It’s legitimate to say that you are focused at growing your user base at first so it’s free with the understanding that eventually there will be a subscription or licensing fee, ads or something that will make you money. Remember, investors want to see a product that is so good that people will be willing to pay for!