We don’t rent to smokers, so not an issue. 

You would be surprised how many people sue their landlords  because of the 3, 4th, 5th, 6th, 7th, and 8 second hand smokers affect their health and anyone that touches the door knobs.

(They have the deferred maintenance you'd expect after a fixed income senior lived in them for several years, plus smokers, pets)So how do I turn 5 straggling houses into a proper real estate portfolio and well-managed rental business? 

Pending how bad the staining - i.e. how long the smoker was resident.  

Are they just "smokers" or drug addicts?

One of my LTR tenants is a heavy marijuana smoker and the smell is invading my Airbnb.

My big 3 requirements for every property. 635+ Credit Score3x rentNon-Smoker

We screen first, telling tenants that we don't accept smokers, we want a 670 credit score or more, that we want them to bring verifiable gross monthly income at least 3x the monthly rental to any showing we set up, etc.

The downstairs tenant has been there for about 10 years and is a smoker.

I have done flips by just painting walls with a smoker that had lived there for 20+ years and it never really came out (we replaced the floors) and were still there.