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.

My husband and I were looking for properties with excellent cash flow to replace our income.

its not like other areas were infill in old established neighborhoods and the build is a Modern design..

Modern plumbing and electrical, brick exterior, water drains away from house, no large trees near house, etc- No one will look after your property as well as you want them to.

Your 1031 replacement has to be worth at least $500K in order for you. 

Same story - The side you live in is her 1031 replacement. 

To replace a plank, you just pull it out.