Roof, siding, windows, insulation, electrical, plumbing, hvac, flooring, etc. 

If you get a deal where you have positive return, then you are somewhat insulated.  

I purchased the paint for another job had $150 in the 2 5gallon buckets of behr ultra, $46 in ceiling paint, 2 shower valves at $55 each, drywall at $64, insulation $60, plumbing fittings at $23, tile was $100, $60 in door knobs, I payed my employees for cleaning all 4 units out, painting 1 apartment and half other, tearing out paneling and insulation, re install new insulation and hang and finish drywall, install 2 shutoff valves and install scald proof valve.

Selling might be tough, building and renting seems like a fair idea, you need nice insulation though.

Refinished hardwoods, new tile in kitchen and bath, old cabinets, older functioning HVAC, insulated.

Insulation, wood, siding, concrete, etc!

My estimates (not including GC overhead) indicate framing (materials & labor) in the $11/ft2, Plumbing & electrical in the $4-4.5/ft2 range each, HVAC in the $3.75/ft2 range, sheetrock & insulation in the $4/ft2 range, cheap vinyl siding in the $3/ft2 range.  

Utilities can be high since little to no insulation, usually old windows, older heating  and even older wiring to deal with.They usually attract a lesser quality shorter term tenant. 

Buffer.It should be a very affordable plan, that provides you with a ton of insulation, and solves most of those common headaches.

I have two exterior walls that need insulation. one particular wall has just the studs and the bricks behind them, so no way of putting sheeting/tar paper/vapor barrier, etc.so, i am thinking of just putting sprayfoam.is there a place online where i can buy supplies so that my contractor can do it?