I’m only an E-3 in the Air Force smh .

I personally like Cozy (super intuitive rent collection) and I just recently heard of a software called Hemlane that seems very interesting. 

Is an app such as Cozy a helpful tool to use?

So 95% of these homes will use heating fuel with forced hot air or a hot water base board with a boiler.

I would consider a multi but if not an option I  wouldn't consider the single an Investment until its producing income (option renting a room or two/ possible air bnb).

. $276/year is locked in for life if I decide to continue to use them after 3.5 years.My property has antique HVAC system that is now > 20 years old, so I'm fully expecting them to break down in the next 24 months, and apparently home warranties will replace the unit if they cannot repair it, which means there's a chance that I will get a new furnace and air conditioning unit (~$5000 value) or at least a cash out value.The other benefit is that I own a few properties, and since the warranty is also transferable, so at any time if I feel like another property has weak systems, I can transfer the warranty over to that property and potentially lower my maintenance and capex cost. 

Seems like a costly luxury to add air bubbles in the tub.

Note that current in-place gross income is approximately $315,000.National Road is outfitted with tenant-paid gas forced air furnaces, electric through the wall A/C units, and individual, gas hot water tanks for each unit.

You can source from Craigslist, FB marketplace, Cozy (and it's affiliates), word of mouth, etc. 

They key is to run it and then completely air out the space after.