Had a structural engineer come out, and he says the house is verging on structural failure - needs 3x as many piers as the original quote, including the entire interior of the house.

These types of studio apartments are designed to have only one occupant, generally rent for $500-$700 per door, and are increasing in demand.  

The exterior of the building is actually in pretty good shape but the interior needs a full rehab.

A good design is also essential.  

I would use the resources like yellowpages, google reviews, ask at a real estate investor meeting or maybe sign up for Angie's list and find the best HVAC company and installer who, stands behind the product, is willing to educate you on the right equipment and provide you a fair quote.Your investment strategy, and what the standard is in your area will dictate the price.I would look for the Highest seer, with a good warranty 10 compressor, 10 year parts, look for one with quiet technology (compressor sound blanket, motor mounts, Air handler blower motor, air flow designed for quietness) and ease of service for the air filter, air handler coil cleaning, overflow float switch, and access or instruction on drip pan tablets/liquid and drain line cleaning.What I look for in a HVAC installer is someone who is proficient and not on the phone or looking through the manual a lot.  

In the Sec 8 Bible they mention using solid interior doors.

This creates a vapor barrier that prevents humidity and odor from entering the interior of the house.  

Most renters are medical staff who work for local hospitals.I have spent the last year remodeling the entire interior.

Please note the information provided below is a layman's definition designed to provide a basic understanding for the general audience.

Arizona statutes are specific in some areas, and using a solid lease designed for Arizona provides the most defensible position one could be in.