We have a lot of foam roofs here in Arizona. I used to work for a foam roofing contractor. Most of these guys are WAY overreacting to the request, probably because it is something they are not familiar with. It is an amazingly tough surface, but make sure you wear soft soled shoes so that you do not put a hole in it. Also when you seal it, make sure that you use an elastomeric coating. You can try these guys, because they are the guys I used to work for. SWD 480-969-8413. They can probably give you all of the particulars on what to do to clean and install the material. As for teh runoff, can't you just wash the house, too? You should have probably included it in your bid. Finally, your bid is probably too low, but I do not currently know the pricing of the materials. This stuff is good, but is a bit more than paint.
http://www.arizonafoam.com/pages/about.html is a link for their construction arm. If you cruise around, there is a ton of good info there.
Scott Stone