The old unit can be disassembled and removed. You can do that yourself with a battery operated drill and a sawzall, and I would not leave it up there.

"Everyone is healthy"...for now. It is just not a healthy situation. As well, the other side of this coin is that your ductwork is likely contaminated too. The fan blows the spores into the duct and depending upon that local environment the spores either grow or die. Who knows. If I was you I'd want to see what is up there. I would not simply have the furnace replaced and be done with it. I think you need to look at the whole system before going forward.

Don't like the idea of relying on pictures. Take a peek yourself.

If confirmed, get some quotes from other than current HVAC dealer. Old furnace and coil should be removed.

IMO

Thanks for your thoughts. I agree I should take a look myself. The attic is tough to enter and navigate but I guess I'll just have to do it. I would love to replace the entire system with a ductless system, but I'd like to buy some time to save up and to do some more research on that approach.

If there is mold, I would want to know what is causing it. There may be a drainage problem. I would expect to see the mold in the drain pan and not the coil I am a little suspicious of this tech.

At 26 years the equipment is near end of life. Start planning on a replacement.

There is no pat answers for your questions but here is information that will help you decide what to do.
Mold on evap coils and in condenste pan is common. The amount is directly influenced by amount of organic matter(dust,lint,ect) stuck in coils. Most mold is harmless and only lab analisas can determine which you have. Need of cleaning coils is common. Effecient filtering pays off in so many different ways including mold, mildew,pet dander and other polutents,it's dumb not to have it. No hvac contractor offers "mold cleaning",the liability would be overwhelming. All hvac contractors routinly clean coils to restore cooling effeciency. Routinly cleaning a coil all but eleminates traces of mold. Duct must be damp to support mold. Damp duct is sign of malfunctioning unit. If a mold remediation contractor is hired to inspect ANY building they find mold and milldew 99.9% of the time. Most if not all will refuse to give you written assurance your mold is safe to live with,regardless which type is present.

The question I would be asking is what caused it. Drainage problem could be a part as mike-home pointed out. Klem1 danced around the nutrient source a little. Filtering is one issue and leaks in the return letting in unfiltered air, from who knows where, is another. Furthermore, what are the humidity levels in the house at this point? If the RH is as low as it should be, mold issues, everywhere, will be lower.

I don't see any health problem with leaving the old unit in the attic. That space is outside of the house envelope. There may be structural or other reasons to remove it. Note that I love my ductless AC, but heat pumps have become an expensive way to heat a home with today's nat gas prices. I live where it is hot, so it is not such a big deal for me.

Wanna see some scary pictures of mold in homes? Check out "My House is Killing me" by Jeffrey C. May. Some molds are more dangerous than others. As far as I can tell, a lot of them are bad for me. My respiratory system is a very sensitive detector. I would not likely be able to stay in your home for longer than a few minutes. I'd not want my family exposed to constant levels of mold fragments blowing around my house.

I would be less inclined to want to replace it due to mold than because it is old.

I would suggest that you at least wear the appropriate facemask when doing your inspection. Take your own pictures and proceed from there.

Research carefully and do what must be done - all of it!

IMO

SR

skeptic that I am...I'd think that he just wanted
to sell useless uv lights. I'd have to see for myself.

even then...I'd want to know what caused the mold
to grow. it needs both moisture & a food source.

rather than $$ & replacement bulb prices for
uv, I'd invest in mastic sealing of ducts & all
plenum to equip connections & equip to equip
connections.
duct/air leakage causes condensation...which is
often the water source for mold if it goes inside
the plenum or equipment.

replacing without proper sealing will just lead
to condensation issues again. and possibly mold.
it takes time for mold to grow..

best of luck.

If there is mold in the coil and the furnace there is mold in the duct work. Sealing the duct, should you stay with it, is essential. Google "doeduct leakage" and read for a while.

I would suggest that you move up your research on the ductless system. The current equipment is shot. The ductwork is suspect. Anything you put into that system, if you really want to go ductless, is a waste.

I replaced my ducted system with mini-splits and I added a Rinnai Energysaver modulating gas direct vent wall furnace as well. Great system. Economical operation. Great zone control and very very comfortable in the home. It is all, "net to the space" with no duct loss.

Thanks to all for the thoughtful responses. Most appreciated and I feel as though I have a clearer path forward.