For reference, we're talking a 24.5' x 24.5' slab 4" thick on grade w/ #3 at 18" on-center rebar with 15"W x 12"D footers, 2" sand over 10mil vapor barrier over 2" sand.If anyone knows someone who is reasonable in cost/licensed (and bonus if they can do the rough plumbing design underneath the foundation), please let me know! 

He said someone can come in and put on a base and cap for around 15K but to do things the right way, putting on new TPO all around would cost around 36k and once we rip up those layers and see what's underneath it could escalate in costs significantly.

You can get material to soften the sound of footsteps as well underneath it.Those stores you mentioned should be able to do the job for you

Home Depot sells interlocking 2x2 panels you can use to create a sub-floor with a small air gap underneath to let water flow. 

(Fannie Mae didn't even put PADDING underneath the cheap carpet.  

I wanted to submit an offer and get my first deal but I didn’t because my contractor said he probably would pass on this one because the foundation was like something he’d never seen before. 2 rooms had concrete underneath and some didn’t.

Removed that old carpet we all love... just when you think you’re going to get lucky and find a nice sheet of plywood underneath, you get stuck with this:Do any of you experienced DIYers, or contractors have any experience with this?

After inspection, sellers confirmed it's somewhere underneath the carpet and they have denied us access to pull the carpet bc they don't know the exact location of it (units are occupied).

Maybe also look into the soil type of the lot?

They are also attracted to moisture, such as from a leaky pipe underneath a bathroom sink or a wet bathmat on the floor."