...completely conventional house in a subdivision...working towards doing the early retirement...plan to sell this house in 9-10 years...everything needs to be up to code before we can sell
Wiley, if I were you, I would take the next 10 years and become an casual expert on your local real estate and build as much sweat equity in your home as you can economically. Maximizing the value of your home can be done without spending a lot of money. Good advice above on your furnace and I wouldn’t cheap out on getting it maintained. There is an inherent economy in keeping an otherwise functional furnace working. I’ve never resided in a house with forced air heating, only cast iron radiators and hot water systems growing up. My knowledge on forced air is that there is an art and science to the ductwork and many ills often reside in leaky joints, badly designed ducts or vents. if you decide to replace it eventually, doing so closer to the date you sell might be a consideration as a selling point. You might consider seeking out an experienced house inspector and have that person go over your house as if you were considering buying it. Document and use that advice s a roadmap towards what you will need to do.
...leave the next owners with a producing food forest and a house that is as green as possible without being so "weird" or high-maintenance that they rip it all out and put conventional stuff back
Keeping in mind your over-arching goals, I would be leery of investing a lot of time and expense into shoe-horning a producing food forest into a terribly conventional suburban neighborhood. Just that in 10 years you will want your property to sell quickly and fetch as good a price as the market will bare. Focusing on maximizing your “curb appeal” .
One last word I will leave here, you mention a timeline of 10 years and that reminded me of a new very green heating technology
I have posted about previously that harvests sunlight, stores that energy (up to 18 years) and can deliver heated liquid on demand and is being touted as having commercial application within 10 years. I could totally see it being retrofitted to forced air systems.