If it's a nordyne, it's factory standard equipment. Can you say energy hog? They do blow nice, hot air, though. Most of the double wide homes come with a 2 to 3 ton heat pump new if you have one installed. I believe the furnace can support up to about a 5 ton phase change system.

To convert the unit over to heat pump operation is about as simple as it gets. Find a cased coil the same size as the furnace, mount it over top of the furnace. Plumb the heat exchanger to a matching outdoor unit. Plumb the drain pan outside. Install a heat pump thermostat and wire the first stage of heat to the outdoor unit. The blower has it's own wire already. Thermostat second stage and/or emergency heat wires to the existing furnace. The outdoor unit can draw power from the same breaker that the furnace is connected to. Draw a vacuum and charge the system. Tada! Easy peasy. Some assembly required.

That sounds like the way to go with the insulated ducts laying in wait.
Imagine its the most affordable way to add a pump giving the situation.
Im sold on it !

Using the existing ducts is the cheap way to go about it. but he won't be gaining any sort of performance improvement. He'll still be negatively pressurizing the house (drawing in a trapping soil gasses, and outside air that needs to be conditioned), he'll have a lot of energy use due to pumping the air through ducts, you have energy loss due to a higher delta-T across a lower R-value, etc... If his ducts were inside the conditioned envelope of the house then sure just adding a heat pump would be a great option.

With the ducts outside the conditioned envelope you then have to look at what the real cost is to provide performance improvements. With that the least cost for highest performance improvement is to bring the ducts inside the conditioned envelope or almost eliminating air leakage and drastically increasing the effective duct insulation. Likely for him the lowest cost way to do this would be a sealed and insulated crawlspace. The average cost to that is around $5,000 and for that you can get a couple of mini-splits installed.

Insulation is expensive but giving the situation and remedy its probably a good way to go.
One more cost but it keeps the ducts where they are and gets the system up and running fast and more efficiently , with luck the crawl space may well of been insulated already as the ducts were.

He feels his house would need 5 indoor units if he went the mini split route I personally like the idea of using 3 outdoor units say one with a dual indoor unit. This way you turn on a split when its needed and let one or two of them do most the work and have the main room heat pumps thermostat set higher then the bedrooms so those splits come on less. Zone control threw the units themselves.