There are rusty strings and then there are rusty strings. Can you clean the strings with 0000 steel or bronze whool and a little WD40?
The plain strings will likely be fine unless they are pitted. There is no way to know for sure about the wound strings unless they are under pressure from tuning. I'd take this track if the seller won't take them back. Check the Ebay description for any reason he won't, such as "Sold As Is"

Punker, Mold and mildew is an issue. If you are that allergic and you have a house that generates mold and mildew in Autoharps you may have bigger worries than your Autoharps.

However:

Mold and mildew need moisture and damp unmoving air to generate. Take those climates away. Put on a mask and start blowing air through your harps, use a box fan, and let it run for a few days.
This next step will sound strange but not really. Go to the hardware and buy some white sand. Sift it to get out any stones or hard lumps.
Put it on a cookie tray and put it ia low oven for an hour or so to make sure it is dry. (People who keep Hermit Crabs know this step, it won't hurt the sand, tray, or the oven) The kind of sand you found in classy lobby ashtrays years ago.

When the sand is cool dump a few cups into the Autoharp and start shaking it about inside. The moving sand will scrubb the mold and mildew away from the wood, the dry sand will act as an astringent and dry the inside of the harp further. Take your time with this step and be thorough. Clean it out with a small shop vac hose. You'll probably be shaking sand out of it for months but you'll kill the mold and mildew.

The out side will clean off with any cleaner just don't be too aggressive. Mildew in the felt is a different worry, soak it in a water and bleach solution. How much a mix is anybodies guess and don't be surprised if you have to reglue bar felts. It may be better to buy new bar.

I've restored Autoharps in the past. 36 for a school district one winter and this will work. Be thorough and aggressive on the inside, careful and un aggressive on the out.

Don