Only time for a quick answer, but I can elaborate later if you'd like.
I've adopted quite a few rescues with heart murmurs, and I've never had a moment's regret. Many were seniors, some were middle-aged. Most of them did quite well and lived into their mid-teens. One senior girl lived only a few months, never had symptoms, and died quite suddenly due to ruptured chordae tendineae.
But......I was an experienced dog owner and was experienced with heart disease, and I already had a good network of specialists (and a good GP vet) who do their best to keep costs reasonable, esp. for rescues.
Where do you live? That might give some clues about potential expense. Do you know anyone with experience with heart disease who could help you with sound advice along the way? You might still want to consider insurance as there are quite a few other things that can happen which can incur more cost than heart disease. (The most money I ever spent on a rescue was to treat glaucoma.)
This experience can be very rewarding; I greatly prefer adopting middle-aged to senior dogs. I've had 12 Cavaliers and 8 other assorted dogs that were all adults when they came to me; some in the 1-3 range, most in the 4-7 range, and several teenagers. I will never, ever have a puppy! (Sing with me to Taylor Swift tune "I will never, ever, ever, ever get a puppy......"
One caveat - From experience, I know not to rely on the health status as reported by any rescue group, and I always have a full evaluation done myself for any dog that I adopt or am considering adopting.
Pat
P.S. It is possible to find a healthy rescue. I have a 10 year old Cavalier that came to me at age 4 (rehomed by his owner who had imported him for her breeding program). He is still heart clear and is SM clear on MRI. He has had only minor health problems to date. My other current Cavalier came off the street with no history at all. Her appearance makes me believe she is a pet store/BYB product. She is now roughly 4-5 years old. She is heart clear and symptomless for CM/SM and has had no problems in the three years that I've had her.