I am a worrier about this stuff too as I lost my first Boxer WAY to young!!!
So I will add this
....If it were me I would want to see hard copies of the heart results. *Make sure its a 24 hr Holter and Auscultation/Echo by a board certified Cardiologist. Some breeders say heart clear after they have only had a Vet listen to the heart. As I said on another thread the other day....breeders are usually glad to show you the results of good health testing!! Again good luck