There can be a fair amount of liquefacation, which is going to seep into whatever's beneath.

clips) and 5/8" drywall will give you much quieter wall assembly (somewhere around STC 55 - 60 ... been a while since I've done the math) while also meeting fire code for common walls.If you want to go further for even less neighbour noise in a higher end unit, you can add a second sheet of (5/8) drywall, but instead of an air gap (which would require shimming and create a conduit for lower frequency sounds), use a sound deadening/absorbing filler such as "green glue" between the sheets.

You have the resources to get professional legal assistance when needed, you could probably show that you had the ability to close (which makes proving you never intended to much harder), you are experienced enough that you can probably stay on the right side of the law and beneath the radar.But there are hundreds if not thousands of posts on this very website from "wholesalers" who have absolutely no idea what they are doing.  

Everyone thinks that a job there is beneath them, but some one has to make my fries.

But beneath the surface- I see there are neighborhoods that are really good. 

Do you have the same layout in the apartment beneath this apartment?

All flooring needs replaced (though there are hardwoods beneath the filthy carpet). 

And the possibility of growing the portfolio to serious money - $50 or $100M level in my lifetime is realistic, although certainly not assured.If we keep doing solid, boring purchases, keep living beneath our means and the world doesn't go to hell then I should wake up one day when I'm about 60-65 and be sitting on $50-$100M.Is it worth it though?

@Michael Herr Thinset is absolutely required beneath backer-board on a floor. 

Desperate certainly but they've been desperate for the longest so nothing is beneath them.