Since it's a newer property you'll have lower CAPEX upfront, other than turning the other unit when the smoker eventually moves out. 

No idea what they vaped but it did leave a residue.Then again I had this new evap coil look like this after 6 months from smokers.

Biggest issue is the owner and her late husband were heavy smokers so that must be mitigated and I think this is why the home has not sold to date. 

The tenant is a smoker and most likely smokes indoors.

All of our guest are pre-screened for identification, pet free, non-smokers and typically only stay for 1-6 nights.

I also noticed on the second walk through that 3 of the apartments had active smokers in them (one of those being marijuana). 

The house was owned by a smoker in the past and we replaced with new carpet and hired ozone treatment company to clean up AC system to remove the smoke smell before moving in, but it didn’t work very well and house still smells.

I have requirements of 650+ credit score, income =3.5X rent, no smokers, and while I've gotten tons of interest, I haven't found the perfect tenant yet. 

The renter was a heavy smoker.

Let them know your basic qualifications like verifiable income that is 3x the rent or greater, no criminal history, no pets, no smokers, Landlord references for the past two years.