Time and material is the best way to bid almost any paint job. Consider how many hours/days of labor times the preferred wage now add all materials that will be needed, include gas, then add a small profit percentage for your company to cover taxes/insurance/etc, 10-15% should cover it.
Added prep work always takes longer and costs more. Make sure to give a comprehensive proposal with all steps and products used written down. Then explain all of this to the customer. If you can think of options that could reduce or increase the cost but add value include these as priced options.