AC units for properties? wall mounted or HVAC?

[Jonathan Taylor]

Hi BP Folks,

I just purchased a fourplex in Los Angeles and each unit has those little crappy in-wall units that hum annoyingly and kick out very little cool air. I am in an up and coming area and nice clean modern units go quickly so I am considering redoing the AC system but dont know much about it. What are your thoughts and experiences with AC systems?

Breakdown, 2 units are on raised foundations allowing for central heating/air but part of the main house (3/2) is a slab foundation along with the studio. 

Any guidance is welcomed

Thanks

JT