Neil, There are a few components to the central AC unit. The Compressor/condenser unit goes outside and the Evaporator (cooling coils) go above the furnace (within the ductwork). If one doesn't have a furnace or the location is not condusive to AC installation then an airhandler (fan/blower) and Evaporator coil are combined into one unit and located someplace like an attic.
When I replaced my two furnaces I decided to also add central AC. Price, efficiency and longevity were high on my list of priorities. I found with some of the brands (Lenox) that parts are proprietary and therefore very expensive, possibility of becoming obsolete or the potential for longer replacement turnaround time existed.
I had the choice from the vendor/installer I chose to go with Rheem or Trane. I chose Rheem as they have been in the industry for quite a while and used a very attractive (to me) technology....Scroll compressors...think Whipple superchargers.
The two units have been in for about five years and have worked flawlessly.
One thing to keep in mind is that sizing the unit's output properly (in tons) is very important. Too little or too big is not good. Also, I chose the second most efficient model as the top-of-the-line cost didn't seem to make financial sense.
