The price per square foot would greatly depend on the location where you will be doing the painting, the conditions of the surface areas to be painted as well as the surface texture of the areas to be painted as well as the method as to how the paint will be applied. So, it would be helpful to have more information from you in those regards.
If you are going to be moving a lot of furniture, as well as doing some prep work, presumably being cleaning, patching and repairs, then you would add more to your price outside of what you would be charging per square foot. Those sort of things would be stipulated in the Contract which you would provide to the Customer.
To be successful at being a painting contractor, you will need, at the minimum, a business plan, at least liability insurance specific to painters, and Contract(s) specific and detailed as to what you will be doing for the Customer as well as the Customer's responsibilities according to that which is spelled out in the Contract(s).
Those would be just a few of the things that you will need to cover as far as what you might need...
There are many that will go around saying that they are painters without really knowing what is involved in the business. It is an art, science, salesmanship and public relations venture rolled into one.
Enough from me...
Please provide some additional information so that we can help you the best here!