Use your Dodd-Franke compliant vendors.

I believe you can owner-finance your first 3 deals, but after that you need a RMLO to stay compliant.  

You always want to be compliant with the real estate laws.And, of course there is Google. 

STRICT compliance with ADA is imperative, and additional 'comforts' including elevators would be almost definitely necessary.

In general, running utility lines to/pouring a concrete slab for the prefab home could be required, and usually isn't cheap where I live.

ADP, Paychex, Gusto, etc) that are priced reasonably, will keep you compliant, and will have that money deposited into your checking account automatically with a W-2 issued in January.Not sure what you mean by 'handling your distribution'? 

Myself I have a building  background and possess journey level skill in almost all trades except maybe doing concrete work and I am also an architectural designer so I perfer properties that can be increased in value through forced appreciation and look for properties where the zoning, set back, and other requirements allow for adding square footage to a building but others perfer invest in turnkey properties already fully occupied whereas I like working with empty buildings. 

I know some of the smell could also be in the carpet and pad (which is being REPLACED), so should the concrete subfloor under the pad also get coated in the same odor-blocking primer?

I'd be happy to look into this for you and get you a concrete answer on how these properties came to get this sign and how (if it is possible) to get rid of it, but I would need to be compensated for my time. 

On a slab foundation the soil should be 4-6 inches below the top of the concrete but if it is not, and the soil gets saturated one of two things (or both) happen.