You can do windows, insulation, solar panels, or anything that is considered making a house more "green".

My mother recently had a rental where the tenant rewired the electrical panels so that their units electricity was on the basement panel (paid by the landlord).  

Here in my area of CA, I would typically expect a cost of around $70 per sq ft of a full gut rehab with new electrical rewire and panel, new HVAC, new copper plumbing, new windows and doors, new stucco, new floors, new kitchen and baths, paint in and out, new base and case plus interior doors and hardware, landscaping, partial roof replacement, permit fees, and misc items.

If you have a property in a high crime area with an old roof and 1970's wallpaper or wood paneling it will be hard for ANY property manger to attract a good resident who will take care of the property and stay long-term.Let me know if I can help in any way. 

I look at every wall, ceiling, blinds pulled down, windows and screens, doors (front, back, and spine), cabinets (open and closed), countertops, floors, appliances inside/out, lights on/off, toilet seat up/down, yard from four different corners, every exterior wall, roof (if visible), water heater, furnace, fuse panel, etc.I start with a walk-through to turn on lights and quickly assess everything and set up for photos.

In a duplex, you have twice as many mechanicals like 2 AC units, 2 electrical panels, and 2 furnaces, which will result in more maintenance. 

He'll give you an idea of the condition of the HVAC equipment, the electric panel, and the water heater.

It's funny you should mention it - I was planning on going to @Collin Schwartz's panel discussion tonight! 

Not sure if city / county code laws do not allow for the homes to be modernized with materials such as fiber cement board / hardi board panels.  

There are many different options when it come to vinyl, the most important of which is panel thickness, which ranges from .040 to .050.