(through the embassy we got air, food, accomodation for AUS$150).Our Earls court 2 bedroom flat ran about £400/week shared by 5-7 guys depending on who was in town.

They should have performed a plumbing air pressure test which will give you a little insight on if there are leaks in the system.

If it's clear I'd take steps to prevent ingress by WDO.

The problem w/ high vacancy is those who may be considering buying a condo to live may not be able to get financing because vacancy is too high.Our rule of thumb for condos- If the bldg has little to no vacancy (its thus desirable) & the HOA rules don't prevent rentals & the HOA fees support the cash flow numbers then hit the offer button.  

In the last 6 months or so: a new roof (I knew it was needed and put it off for about a year), 3 new A/C units, one water heater, and one house needing a new air handler (estimated cost $1200).

Depending on what we decide to do with the current floor plans - the kitchens are tiny (upstairs is 8x8) - we may take all external walls back to the studs and properly insulate and air-seal the envelope (likely with closed cell foam as I expect there to be limited wall cavity space).  

Black mold coming through the air vents.

In other words, you don't want to rent to people you are up in the air about whether they'll pay.  

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.