I have had barbie style dresses fit Jem dolls but not ones that have tight skirts as Jem has a wider waist. Trousers are no use. I ended up making outfits for some of my dolls, because Jem's proportions are what they are.
Also, though, a tip. Some of the vintage Jem clothing is not that expensive. Some of the outfits are fairly common, and if you hunt around in the barbie fashions section on ebay and such similar sites, you sometimes find Jem stuff in with barbie stuff. In any case, getting basic clothing for a basic first edition Kimber, Aja or Jem is not that bad. It's the second year dolls and fashions that particularly get pricey.
Pranceatron vs Jem is a great site for fashion part ID if you want to go hunt for them. Also clothing for a doll line called Darci will fit Jem and that is often not too expensive. I think there is another doll line that works too, but unfortunately right now I can;t remember.
But yeah, MH is definitely impossible.
I actually have some spare Jem dolls but aside from me being where I am currently, the shipping would probably be bad because they are quite heavy dolls...