The heart murmur and panting has me thinking he's probably got heart disease where it's progressed to aortic stenosis. I think this because I have a bad aortic valve, and the reason doctors first found out was through hearing a murmur, and now it's progressed where I'll need valve replacement, but ususally the decision to have surgery is based on side effects of chest pain, fainting/dizzyness, or shortness of breath.

Since you say his breathing problems start when he gets stressed plus he has a murmur, his aortic valve's probably wearing out - especially since he's 17 and in "senior" range for a kitty.

Our indoor/outdoor kitty had to be put down at 17 (which is pretty long for a cat that goes outside). It's hard, but all you can do it make sure he's comfortable. You'll know it's the "right" time if he's no longer drinking water or interested in food.