Well, neither of my children was a heart child, but from being a mom and working in a hospital and as a teacher, I can tell you that a lot depends on the parent and the child. I remember the first time I took my daughter to the dentist, I wanted to go back with her. The nurse suggested that I stay behind, but that she would get me if they needed me. I knew good things about this dentist and his staff, so trusted them. I noticed a poem in their waiting room that said something to the effect that nervous hovering moms make their kids nervous. Whenever they needed shots, I explained that it would hurt (not just like a mosquito bite which is what some nurses will say), but the pain would be over quickly and the medicine would keep them healthy or cause them not to feel the worse pain (like a filling). When my son needed surgery on his broken arm at age 7, I explained the IV and the procedure in kid friendly, but accurate language, ahead of time and he did fine. Was I nervous? Yes. Did he know it? No.
Your plan sounds good, but if he is already nervous about it, be careful not to overdo it. Talk to his doctor and the other care providers about his fears, but do this when he is not around. If he thinks that you expect him to be scared, he will be. I don't know about the buzzer thing because I think it places too much emphasis on something that is not really that bad. Yes, it hurts, but not as much as a skinned knee.
I once attempted to take blood from a 10 year old girl. The mother met me outside the room, told me the girl hated shots, and wanted me to "sneak up and do it fast." Apparently this is what they did with vaccines, but I explained that this was impossible because of the tourniquet, finding the vein, etc. So the mom walked in and said, "Well, let's get this over with," grabbed the girl's arm and held it tightly. Needless to say, the girl went into hysterics before I even brought out my needle. The ER doc forced the mother to leave the room, calmed the girl down, and I took her blood without a problem. The mother was livid that she was treated that way. I was 19 and this experience taught me a lot!
One of my former first graders had cancer and had been in and out of the hospital since he was 4. I was amazed at how a young child could go through those treatments. His mom said that he never cried, and she once overheard him talking to another cancer patient in the hospital. He told the little boy very matter of factly, "The needle will hurt and the medicine will make you sick, but you need it to stay alive. You don't need to cry because staying alive is a happy thing. When you get out, we can play again." She said that she had told him something similar before getting his first chemo treatment, and it obviously worked, because now he was a pro and passing on advice to the newbies.