In that case I would purchase four window style air conditioner/heaters to mount into two of the windows per unit.That means each apartment will have two units they can control and this will be more than enough to heat and cool 1000 sf or less providing you purchase them to accomadate the correct square footage.The pros and cons of going this route:Pros:Cost.

Air Conditioner ?

Air Conditioner?

The 2/1 is available to rent - paid $7,001 for it, fixed windows, replaced stolen air conditioner, paint, cleaning, and minor items.

Air Conditioner ?

Just the basics, little shampoo, conditioner, soap and lotion bottles in each bathroom, salt, pepper, cooking spray, coffee grounds, etc..

My tenant moved out a couple of days ago, and when I walked through the unit, I discovered that he had removed the curtain and curtain rod, detached the room air conditioner, and caused significant scratches on the hardwood floor from the metal bed frame legs.

I have also seen cages and bars over air conditioners down here.bullet holes over cribs? 

I haven't rehabbed a house yet, but when doing my math, I have found that tile floors, new cabinets, new windows, and new appliances are the most expensive.This is assuming that the air-conditioner and roof doesn't need repair.

The window unit you had when he moved in, should be good enough, unless it does not function as it did.However, there are larger air conditioners and there are fans to help circulation from the other rooms.