Parts of the ceiling, sheet rock, kitchen cabinets, and floor were torn out and it looks like there haven't been much work done on the mold issue for quite a while (can't confirm how long, just guessing). 

Different situation might be a Rock Quarry.   

and you have a lots of rock and volcanic rock here.. 

This often times means you gotta turn up the rock n roll, put your head down and get laser focused.  

And, your first flip, no less...Rock on!

You rock

If you try to use a paint brush/roller it will feel in the very small texture on the sheet rock which is what makes it noticeable.

Just thinkn out loud.

His previous experience includes purchasing buildings (like an old body shop) and sand blasting the interior, polishing the floors, redoing some sheet rock and leasing to tech companies looking for an open floor plan.

I've been out of commercial construction for a while now, so I may be mis-remembering, but here in Kansas City you could get new asphalt pavement laid (including base rock/fine grading) for $50 to $70/SQ ($5.5 to $7.5/sf).