Replacing air conditioners in a small apartment building

[Skip Frisz]

i am looking for a recommendation for an AC system to cool small 2/1 apartments.  I want to install a system to replace the ones in the attic as they reach the end of their usable life. 

Here's some background.  This apartment building has 12 units and has some very old central air conditioners that will all likely need replacing at some point.  The apartments have 6 units on the first floor and six units on the second floor.  Our challenge with the 2nd floor units is that they were installed in the attic a long time ago and are very hard to replace and costly to service.  I would like to continue with a central air but with a system I don't have to install in the attic so installation and repairs can be more cost effective.  Our property manager recommended a system like a PTAC (Similar to a hotel system) but you would have to cut through the exterior block wall which I'm not a fan of.  

Would appreciate any suggestions.  

Thanks,
Skip