One of the reasons to enlist the services of a property manager is to insulate you from the tenants.

Blow a foot of insulation in the attic...6-800$.

The architectural design to meet sound insulation requirements as the following:   - 50 STC (Sound Transmission Class)   - 50 IIC (Impact Isolation Class) The apartment complex to have flat roof area of 500 -700sq. ft. that is ready for photovoltaic panel installation.

Flooring tiles, siding, insulation, etc...Lead paint: Anything build before 1978 could have lead paint.

I have a old house that has knob and tube wiring in the attic that I need replaced before I insulate it.

These are all questions whose answers will help you chart the way forward ... there is no one size fits all answer.From an operational perspective it is always pleasant to know that your building envelope is relatively efficient (insulated and air sealed); that building systems (electrical, plumbing, HVAC and communications are up-to-date and dependable) and that you have the ability to put accountability for resource consumption in the hands of the tenants.  

In addition to how you work the roof line, you also have to be aware of how you are going to do the insulation and venting. 

I have no clue on insulation, windows, fire alarm, CO detector, fire retardant door thickness.

Some are even building HUD homes with 2x6 exterior walls, radiant barrier roof decking, a full house moisture barrier, low-e vinyl windows, extra roof/wall/floor insulation and high efficiency HVAC systems.

Structurally the house looks sound will need all new insulation, electric, plumbing, and probably plumbing.