I'm curious how you write the insurance money into the sale. My thought is the re-appraisal will set the value of the home without the buildings. I think the seller will have to relist it at a lower price and the seller will pocket the insurance money.
The appraisal most likely won't come back lower by the replacement cost of the buildings but if the seller has insurance he likely has replacement cost. Therefore he would pocket the insurance and the appraisal would come back slightly lower and he would get almost as much for the property and the buyer would be out the buildings and money.
I would make the seller either reduce the cost of the property by the replacement cost of the buildings or put ALL the insurance money into building a building back and write into the purchase contract that the buyer got to choose the design of the building and could put forth extra to at to the build of the building back his way.
But I am a cynical a-hole, I wouldn't give my advice on such things very much weight.