Some rotting sills around front of house and framing members-Concrete slab shows evidence of settling -Galvanized steel pipes in the house-Moisture in the basement (seller stated this was fixed)-Vermin activity (seller stated this was fixed)My questions: -Is it worth pursuing this deal? 

Are you talking about protecting it from moisture or from impact?

These are some things to consider, because they will both contribute to water penetration.

This is similar to paint you apply to the outside of the building which creates a moisture barrier.

It's about the molecule size, and stain that has bigger pigment size won't penetrate the wood grain.  

Another reason the range is so wide is because contractors wont really know what kind of work this project will entail until they see behind the walls and look into plumbing, moisture issues, electrical, foundation, etc. 

The house is in a terrible state of disrepair and I am not sure what else will come up once walls come down and floors lifted - as it is many of the walls show signs of moisture and I am worried there may be some mold. 

Not a lot of course but puddles here or there due to moisture in the ground.

We really need to get that moisture out of the bathroom. 

Some foundation companies can provide this in a less official manor but as long as you have the map then you can see what needs to be done (example picture attached) Water Flow - usually the foundation has shifted because moisture is not being properly regulated.