A/c Mildew question

Hi folks, need a little advice here. I live in florida and hired a home inspector to check out my house which I will soon sell. He observed some mildew or mold on the sides of my a/c closet unit inside my house. It is a combo unit meaning it is electric a/c and a gas furnace for heating. He tested the furnace portion and noted that it was difficult to start. I told him that every year I have the gas company come out in the fall and they do a test, service, and inspection on it, and after they do their magic, it works good for the winter.
The inspector said due to the mildew and the hard to start heating, I should replace the unit, probably with an all electric one. He said due to the design, it cannot be disassembled easily to see what it looks like on the inside. I believe it is around 13 years old, but it doesn't look very old or in bad shape.
I would rather not replace the unit as I already have to lay out some cash for a new roof. Can the mildew on the sides be treated & cleaned? It is on ducting I believe, rather than the unit itself, or perhaps on some kind of insulation covering the sides of the unit. I believe the brand is Tempstar, but I could be wrong.
Thanks for any advice.
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Home inspectors are the biggest rip-off there is... call a licensed, insured, professionally trained, HVAC technician to do a complete service and cleaning on the system. Call family, friends, in-laws, out-laws, folks you go to church with, etc to see what company they use to service their systems and why.

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