HVAC Options in Duplex

[Michael Leeson]

Any experience or suggestions appreciated! Closing on a small duplex this week. 

It has a single 30 year old furnace and one thermostat. Here are options I've got so far:

1. Replace furnace and use window AC units - $4,200

2. Replace furnace, add central AC with zoning - $9,100

3. Add ductless mini-split units for heat and cool - $10,500

Would any particular option have a greater ROI?

Thanks for the input!