You'll need to know what needs to be painted, and what does not need to be painted. Your point of contact will need to be VERY SPECIFIC when you ask him these questions. Go around the entire property, with him, and make sure everything is clear (what is and isn't getting painted). The last thing you want is to go on the job, and paint something that wasn't supposed to be painted, or not paint something that was supposed to be painted, that you didn't budget in your proposed price (waste of time and material).
Then, after that's all cleared up, get the total surface area of everything being painted. This could take an hour or 2, but is the best way to get an accurate estimate, and create a trust-worthy proposal. After you find the surface area of all the surfaces being painted, apply a production rate to each surface (hopefully, you have a solid idea of what your production rates are based off your experience), and also material cost, based off the material's coverage rate.
After you figure that out, put together a scope of work in the proposal that clarifies EVERYTHING (what is and is not getting painted, what's to get covered and protected, when will the work be completed, who's providing the lift, dumpster, etc.).
Specifics are key. Don't be vague with your proposal, or else you may end up having to do work that you didn't budget for, and end up doing everything for free.