I would have a contractor come out and evaluate the whole system, ductwork included, as part of an estimate. Then I would have the contractor determine if the current furnace is properly sized according to a Manual J load calculation, which is the proper way to size systems based on total heat loss and total heat gain. It is sometimes hard to find a contractor willing to do this, but it is important.

A heat pump would likely be a good idea if your climate is rather mild (even heat pumps can heat down to fairly low temperatures).

I see three options:
-just add air conditioning and keep the furnace (assuming the furnace is sized correctly)
-add a heat pump with the furnace for a dual fuel system (again assuming the furnace is sized correctly)--whether or not this is worth it depends on your climate and gas/electric rates
-add a heat pump and remove the furnace

Your furnace probably doesn't have a variable-speed air blower, which I would recommend.