I went there on the 27th of December, and it was extremely cold in the house, so I looked at the thermostat, and it was on 64 degrees. 

Items I've had to deal with:Can each unit separately control their heat thermostatically?

In buildings with central heating systems we use programmable thermostats which allow a "hard" max (heating) or min (cooling) temperature to be set.  

I can already feel all the text and calls it and I will receive from the tenants complaining that the thermostat is either too high or too low.

Wifi lock, front camera, wifi thermostat.

Does an exterior door let cold air in next to the thermostat?

For investors with slightly different goals than you/Ben, the “$250-$300 is all that works” statement simply is too simplistic.To start splitting hairs, ROC isn’t the only metric that matters- time and effort matters too- popping in a high tech Alexa enabled thermostat is a quick repair, but adding a backsplash takes a decent amount of time, even if the rental premium and cost were the same.

If you pay heat they won't conserve, so be sure to install temperature limiting thermostats. 

Then you will need $500+ in additional materials, $100 new thermostat and they will probably get about $1000 in profit for running business so that will put you very close to 6k.

Things they need to handle are the regular home maintenance items-  light bulbs, sprinkler heads, ants, spiders, spoke detector batteries, batteries in thermostats,  as well as anything they cause- like plugged toilets, etc.