A quick question here, when I am building a new bathroom with a shower do I need to apply the redgard material or just the purple or green moisture resistant Sheetrock is enough? 

do a full inspection before purchase, i did not just did and ARV appraisal, and 4 point found moisture in the roof that we did not find.

The moisture was trapped under the plastic, so it was much slower.

Otherwise the brick will suck moisture out of your stucco decreasing its strength.

A bigger issue to consider is where the moisture is coming from and how to reduce it.

If you decide not to tile, be sure to at least use greenboard/moisture resistant board, as most older homes have regular drywall in the bathrooms.

And the next time the tenants were gone I would hear water running in their unit and have go in to investigate in every corner of the place for moisture.

I think I may have a moisture problem at the back door so I am planning on replacing the door.  

it is a little uneven, partially cracked and I see moisture is getting there on the edges close to the walls. it is not swimming in water but definitely wet.

One other point I have is to make sure the bathroom has a good proper sized exhaust fan, steam moisture does more damage then anything, I also try to have the overhead light wired with the fan so when light is on, so is fan, saves those tenants who cant get the point of the fan or that it actually needs to be turned on to work.My home bath I installed a push button timer switch that has different settings ( 10,20,30 etc) saves the kids from leaving it run all the time.