I can build an addition for about 120k that contains two "units" of 2bd/2.5ba each with lots of premium features (9.5' ceilings, acid-stained colorful concrete floors, radiant heating throughout, central air, LED lights, granite countertops, subway tile backsplash, carrara marble porcelain look-alike bathrooms, etc.)

I replaced a 2 1/2 ton air handler and 2 ton condenser at a rental last Oct (builders sometimes did that to squeak out a little more cooling than a 2 ton but less than 2 1/2 and save a little on the equipment cost).

They were "snow birds" and when they came back to their Palm Springs condo, mold had taken hold up the sides of the drywall in every room!

Like yourself I spent a lot of time reading posts and listening to Podcasts (all of them now) and I plan to invest in the spring.  

In fact, it's enough difficult that we try to have our leases timed with spring or summer.  

You both decided to air your grievances on this post when if you felt it bothered you so much, perhaps you should have passed it up. 

Do an air pressure test on the plumbing.  

Much better units mechanically, they move more air, and they last forever so far as I know.My original thought was that they would be easier to repair or replace later - but so far I have no experience in that regard.

After checking to make sure there are no openings between the ceiling and attic, do you think some small, air-driven fans near the attic vents would help prevent condensation from building?

We are hoping to buy 3 more houses this spring.