Not sure why you think an RSPCA vet would be able to give better advice than scoobydoo's own vet.
I bred a puppy with something called PDA Patent Ductus Arteriosis, (sp) which can affect any breed and is not inherited.
This puppy was given away to a friend of mine who knew the problem and she lived a normal life until she was five. She was unable to be operated on.
If you find out what is causing the heart murmur and what the vet says is the likely outcome I am sure you will find lots of information on the internet. I would not consider surgery unless there was likely to be a very good outcome.
If I had bred this puppy I would take it back and either refund or replace it, which I know is difficult for you but you have to be realistic and the operation will no doubt be very costly. If you are going to end up having to either have her put to sleep anyway, or maybe losing her very young, it might be much better all round to let her go back to the breeder before you get too attached to her.