I visit my property monthly when I do preventative manintenance - test smoke alarms, test GFCI receptacles, flush WH T&P valves, change AC filters and flush condensate drains, feel the bottom of sink drains, blow leaves off the roof etc etc etc...For a while I did the "call me when you need me" thing, until one day a tenant moved out after 1 year, and I went to the AC closet, and I saw 11 filters untouched.  

I've heard AZ has been pretty warm lately...Also, for regular maintenance, you definitely need to wash out the condenser unit, any dirt, algae, damaged and missing fins, will affect performance.

This is a long and convoluted story, but I'll condense it to the basics. 

If you put foam board in, make sure you peel of the sheet plastic, you'll get condensation that may run down dripping off in the wall cavity causing problems.The much better solution would be a metal roof over the old.

If there is moisture in the concrete condensation will show on the underside of plastic.

Since apartments are much smaller than homes and there size is condensed the square footage of my roof is 12,000 sq ft x $400 = 120 squares x $300 (because it is a flat) = $36,000.

AC unit needs condenser clean & insulation around condensing linesThank you guys in advance! 

one contractors recommending belowClean blower wheel inside furnace around 345$ ( he says cutting air flow efficiency by 20%)To replace blower motor capacitor around 125$ (says reading should be 5.0 but it is 3.9 may fail any time)To replace condense capacity 135$(says reading should be 5.0/40 but it is 4.8/37  may fail any time)What are the worst numbers until which these capacitors work before worrying about replacing them?

Plus get a speed door.If A/C check for Condensation at key areas.Would still recommend you add a Dehumidifier system to work with both your Heating and Cooling systems, Winter and Summer.

It will get condensation between the insert and tub over time creating mold.