I would generally agree, but Michigan does get pretty cold for a good part of the year. He would have to have a unit that does very well in very cold weather.

I agree. I say this year, throw together an outdoor wood fired boiler out of recycled whatever, and run it to a radiator of your choosing with some insulated pipe. Save money, chop wood, and seal and insulate like crazy to maximize heat retention. Buy an IR camera or thermometer and let the hunt begin.

Later, a long-term solution could use a zone or two worth of (inverter) mini-split units placed strategically or a retrofit could be made to the central air system using a high-efficiency split ASHP. Unless the central system is in perfect shape, I would begin to plan its removal or remediation. Since natural gas is already there, it can serve as a backup on those awful cold spells when the heat pumps are not as aggressive.

Right now is a good time to get a baseline measurement on the heat load. If the furnace is the only thing that runs off gas, it's easy: just check the gas bill. If not, some accounting will need to be done as to the run time of other gas appliances.

As to the payback or ROI factor, remember that the tide is turning on eco-friendly building. People are now willing to pay more for a house that demands less heating and cooling, and that uses more durable/natural construction materials. Sometimes, considerably more (like double). So whether or not you'll live in the house forever or not, you should still aim high when doing home improvement projects.