More simply put, you'll probably need to install a new support system to replace the wall you're tearing out.

If you do this enough times, you can replace your W2 income and then go from there as strictly a real estate investor.

Say that HVAC system costs $7000 to replace.

In setting up the utilities, I had the city come out and replaced our trash bins since we as owners are responsible. 

We asked the seller to make repairs we did not want to make ourselves, as well as asked for a few thousand to repair the roof, get electrical work done, and replace some appliances.

The newly renovated rental condos I bought had a landmarked roof requiring a million dollars to replace and each owner is assessed $10,000. 

Sounds like you already have some knowledge so will just answer your direct question.The replacement property needs to be like kind exchange.

I’m replacing window units with mini splits in a 6-unit apartment building where there is no space for HVAC ducking.

It's empty right now and I'm leaning toward replacing windows since they seem to be old as the hills (people walking by simply talking....not the usual screaming in the neighborhood) can easily be heard when inside. 

Replace any window, glass, exterior door.