A seller disclosure form can give you an idea of potential problems, but it should never be considered a substitute for a professional home inspection. Sellers may overlook problems, and often not be aware of them. It's smart to use the disclosure as starting point, but hire a pro to get the whole story.
Most exterior paints will cover about 400 square feet per gallon for one coat. But adjust for waste factors like the type of surface and equipment used. If the surface is rough or porous, add 20% to the total gallons figured. If siding is narrow (4 to 5 in.), add 10%. If on a corrugated surface, add 33%. Also add on a 10% waste factor if using brushes or rollers, add 20% for airless paint sprayers, and add about 40% for air paint sprayers.