Okay...
I want to say- I am not a vet. I am just someone with some kitty experience, but others have more kitty experience then I
There are often innocuous heart murmurs that kittens grow out of- but I think they are usually gone before 6 months of age. This is a condition that is also common in human infants and when someone is worried about a minor heart murmur when their kitten first gets home- I'd tell them to give it time and re-check at 6 months.
At nine months, I'm not sure the kitten will 'grow out of it'. I wish Per was on here to give a better definitive answer on this one. It is NOT the breeder's responsibility, per se, to get a cardiac ultrasound. Right now- she has done the responsible thing and informed you of what her vet has found- now the ball is in your court.
What do YOU want? If you think a cardiac ultrasound should be done before the kitten is shipped- then I think you should inquire as to the possibility of having that done. I WILL tell you- cardiac work-ups are NOT cheap. Typically, a cardiac work-up can be well over $500 (I am thinking more so towards $1000 locally- but we have higher vet bills here then most places).
No one can even begin to guess about your kitty's future condition without a further work-up.
Ugh- I am so sorry! I wish I could be a bit more definitive on things, but it's very hard. It's going to come down to what you are comfortable with and what you feel you can deal with. YOU DO need to talk to your breeder, however, about how accepting a kitten with said condition already diagnosed will effect your contract.