I will spray mold killing agent. 

The one I recall the best was for a 150 lot development we did just above Beale airforce base  ( home of the SR 71 and U2's which are too cool to watch )this was in the early 90's plant was going on 250k then and then the spray fields were not to bad..

@Nate Miersma - you could also consider spraying kilz over everything and adding 1/4" sheetrock on top of the existing sheetrock.

Check for meth use (can of spray starch works I was told by a MH investor).

After you have sprayed that and let sit then you can either sand or just paint.

I would spray 2.5" minimum in the stud cavity to meet the 2012 energy code of R-13 in walls. 

Would spray some weed killer down and have some churn the soil so it's dirt, Maybe lay some stones down and make a stone fire pit?

He wants to have a professional come out and spray.

I'm guessing I will have to demo out more drywall and then just spray down the studs with some bleach/water?

Yard, quarterly pest spray, lawn doctor, termite bond, were standard expenses.