More specifically I am torn between water or dry lot.

After that, you are replacing dry wall and possibly studs to get rid of the existing mold.Do you have pictures of the damage?

If he has buyers, the only advantage they have is knowing it's a done deal before the ink is dry, but if there is some ten day rejection period, that slows down the transaction more than a straight listing.

Right now, you're facing the same thing a lot of other listing agents are facing: demand is drying up as buyers try to make sense of what the market is doing.

Do each room and let it dry.

After it's all dry, a regular painting 1-2 coats should make everything look new.

Throw them out or, take a power washer to them and let them dry in the sun - lol.

How do I go about estimating the cost of all the lumber needed, roof material, dry wall, siding, flooring etc. any advice helps,thanks Pat 

If you decide to hold the property and they've done a bunch of stuff to the house, they will hang you out to dry.  

But there is also a ton of dry powder waiting for opportunity in multi-family.