With some proper planning and the funds to lock in materials early, you can have an efficient build, but certainly not fast.

I would also focus on landlord friendly states if you are planning to rent.

In a  Dplus area, 2nd floor unit, considering installing LVP. 

Licensed with permits ~$3,250.HWT standard gas single family low efficiency without exhaust retrofit $900.Sand and stain hardwood floors, natural stain $3 per sf.Paint interior walls, ceiling trim and doors 3 tone paint scheme 2 coats $3.20 per sf (floor area cost includes paint)Click lock LVT flooring install overlay inclusive of prep and quarter round $4.50 per sf (materials and labor)Asphalt roof installation 3/1 - 800 sf bungalow with 2 layers of shingles - ~$7,900Standard porch cap replacement with 2 new precast concrete steps ~$2,500Used stove or refrigerator delivery and install $350New stove or refrigerator delivery and install $HD model specificUse this as a general guideline. 

., new flooring, updated bathroom/kitchen fixtures)-Energy Efficiency Upgrades (e.g., solar panels)-Accessibility Modifications (e.g., ramps, widened doorways)-Luxury Items (e.g., pool)-LandscapingNot Acceptable Renovations:-Commercial Use (e.g., turning a residential property into a commercial property)-Temporary Structures-Non-Residential Buildings (e.g., barns, stables)FHA 203(k)Minimum Down Payment: 3.5%Minimum Credit Score: 620Minimum Loan Amount: $50,000Maximum Loan Amount: $524,225Occupancy Types: Primary Residence ONLYUnit Maximum: 4 UnitsAcceptable Renovations:-Structural Improvements/Reconstruction (e.g., adding rooms, bathrooms)-Cosmetic Enhancements-Eliminate Health and Safety Hazards-Energy Efficiency Improvements-Major Landscaping (e.g., grading, tree removal, adding walkways)Non-Acceptable Renovations:-Luxury Items-Commercial Use-Temporary Structures-Non-Residential BuildingsBoth of these renovation loans are similar in many ways, but the key differences are:1.

There would potentially be some weight issues with the 2nd+ floors of some shopping malls, depending on what the potential tenant wants to store in the upstairs unit. 

The property is livable, but I am account for a pretty significant turn in 4 - 7 years of paint, flooring, and any miscellaneous repairs that the inspector could have missed since it was occupied.

I think you are looking at the wrong places, insulating the floor wouldn't change much. 

In the end, it would have made more sense to hire a couple handymen for a week or two than me spending 3 weeks installing flooring, base and case.

I'm thinking long term rentals will be a good retirement plan.