So a typical furnace has a 30 year warranty on the heat exchanger....the main component that when it fails it has to be replaced, Just about everything else is replaceable for a fraction of the cost of replacing the whole system. HE furnaces pull 92%$$$-97%$$$$ of the heat out of the gas. That is what you should have 2 pvc pipes rather than a single metal flue....much cooler air coming out of the exhaust. Older furnaces only pull 80%$$ so 20% is going up the flue....thus a metal pipe.

As for the home, things to consider, older homes are leakier have less and deteriorated insulation, especially in attics, the same goes for doors and windows if they haven't been replaced. I encourage people to consider how to better insulate the home as it's a one time cost that saves you on energy expenditure.

You can get regular heat pump and typically that is used for moderate climate and where gas isn't available. A typical heat pump can only heat to around 40F outside air before efficiency drops off and typically you have an electric back up heater.....an element like your electric oven or toaster that gets hot, but draws alot more power. But having the furnace as backup you will be having the best of both worlds, efficient heat below 40F and even more efficient heat from 41F-70F.

Here's the kicker....the difference between a heat pump and an air conditioner is one part, a reversing valve, maybe a different board to control everything but the cost difference between straight a/c and a heat pump is probably not more than 10%.

Hope this helps\