You may want to look into technology as well like new smart devices such as thermostat, lights and so on.

The thermostats, for instance, you can manage from afar and most utility companies give you instant rebates on them. 

Estimating ~$40k to get electrical up to code & install new HVAC (currently one thermostat controlling all units).

Try to limit them as much as you can be limiting thermostats, water savers, motion detector lights and low wattage bulbs where reasonable.

Remote capability to set the thermostat or locksets is also good.  

.- Smart Thermostats.- Bushhog and mulch overgrown pasture.- Paint inside and out.- New front porch and new deck/patio on top of garage.Hoping to sell it fast. 

Hello,I recently purchased a duplex and realized (...) that there is a single gas line and a single thermostat.

The current owner installed 3 new HVACS: one HVAC for 3 units on the 3rd floor, one HVAC For 3 units on the 2nd floor and one HVAC for a larger unit on the ground floor.So there is one thermostat for each of the above floors and they are simply set to 70 degrees year round.

There are available thermostats which will control both the furnace and heat pump (or air conditioner) allowing the heat pump to provide cooling and stage 1 heating.

. #4 I probably would have used heat tape with thermostat instead of cutting a hole and letting heated air into the attic.