[toomuchtodo]

The only part needed to make an AC condensing unit a heat pump is a reversing valve (which can change coolant flow direction based on thermostat input). The cost for this valve is $100-$300. There is no reason to not make every unit a heat pump by default, considering the minimal cost and that you're amortizing the cost of the system over 10-20 years.

With that said, depending on your latitude and climate, non electric backup heat might be necessary (to your point, a fossil gas furnace). Depends on your home envelope thermal efficiency (how long can it maintain heat without additional input or solely with solar input during the day), electrical utility reliability, etc.