If you buy an empty condenser, you then have to add refrig. 

The beauty about this system is you don't have a condensing unit(s) outside the building, nor do you have all the refrigerant lines running to it.This assumes of course you have a window large enough for the unit to fit in (this one was 45"H x 29"W) and enough floor-to-ceiling height to run the duct between rooms. 

and don't get me started in the deep south about condenser units.. there is a reason some company came up with lojac's for condenser units.. bottom line there is simply risk.. how you mitigate it or as you state circle the borders is whats important and in my mind you do that by not setting yourself up for this buying the lowest price properties in a given market.. that simple step will save you time and grief.. leave those for the locals or the guys and gals that do it for a living.. you don't want to be a CA landlord busy at your job worrying about your low value asset rental.. that's no bueno . 

The AC, especially, a wall unit, will gather condensation in the bottom of its base, and can sometimes produce a smell of mildew when the unit is in use.

You air handler may need to have the coil cleaned or the outside Condensing unit may need a good cleaning.

Your condensate line should drain freely.

Also will gloss over details because it's a pretty long story, this is condensed.

The outdoor condensing unit alone one a multi zone ductless system is often the same or more that a furnace, AC coil ,and Outdoor condensing unit combined.

Fog/condensation was observed in this thermopane window.

Water got in from the gutters and rain hitting the siding face, where it couldn't escape.combined with condensation formed on the inside wall cavity from temperature differences between an air conditioned room at 75 degrees and an outside wall over a 100.