I have narrowed it down to a 3-4 flat, or if I do end up employed in property management and gain tenant handleing experience possibly a larger complex.

They probably consider the RE income like self employment income, they need to see two years to establish stability. 

My tenant is breaking lease, because he found a job in another state.His new employer will pay for the 3-month penalty as part of the relocation assistance, but requires some sort of invoice to show the total amount due.I'm a private landlord, and don't have any official looking invoice for this purpose.Does anyone have any suggestions on how to tackle this task?

I'm not an expert, but in my view, small towns with relatively no industry or large employment base do not typically experience any significant growth.

Luckily I'm still employed, but spouse fulltime caretaker.

Is there a diverse economy and diverse employer base?

From the county records of how much a property transferred to, to demographic and employment data, rental rates on zillow, trulia, rent-o-meter, street view, birds eye view, etc etc etc.  

I work for a company who employs about 43K or more employees worldwide.

Many students aren't going to be making that much money a month - I didn't when I was employed in college - but I still had all my rents paid on time.

The relocation need not be employer mandated to qualify for this exception.