My goal is to build a portfolio to supplement or even early retirement.

Indiana is great for primary home owners with low property taxes but as an investor, you lose the homestead (homeowner), mortgage and supplemental exemptions.

Have you looked into mini splits for supplemental ac, this would be my go to to at least explore. 

The credit check gives you their payment/debt history from the past 24 months, and the background check gives high-level crime, previous address, etc. information.I supplement this with a thorough interview (I'm house-hacking and will have to live with these people though), and call their previous landlords/references/roommates to confirm that what they've told me is true.

We actually got the idea from a friend in Denver who was using Airbnb in his basement to supplement income.

We are raising $50K to supplement his rehab loan on it.

To supplement this - the correlations among stock market (dragged down in partt due to oil and gas debt repayment issues), my job income servicing oil and gas companies and the local housing market have a potential to be a triple whammy but also a real opportunity to buy cheap.

It took me 30+ meet ups with all different agents until I finally found my guy, and I'm still seeking another agent to supplement my search.

Through passive income, I hope to gradually supplement my salary and replace it completely within 5-8 years.  

My client has supplemented his retirement income, the buyers have good income but some credit issues that prevented them from traditional financing.