If a Colt King Cobra is not worth $550 now, it will be soon. While not of a Python's finish level, they are very nice revolvers. They are also about as durable as granite.
The Colt King Cobra is a heavy duty revolver, and it is relatively easy to shoot with hot loads. Hot loads will not faze it.
As Zebraranger said, you will want to use snap caps for dry firing. This is not so much because the firing pin is weak, but because if it
does break, the gun will have to take a trip back to Hartford to get it fixed. Replacing the firing pin requires a certain jig to remove the old one without warping the CKC frame. Colt has the jig. You local gunsmith doesn't.
I recommend you and your friend meet up at the range. Bring some .357 magnum ammo, and shoot the gun. If you like it, buy it. If you don't, pass on it. After all, the worth of any firearm is only measured in how much it is desired by the buyer, not what the Blue Book says. If you do not desire this revolver after shooting it, it is not worth $550 to you. Period. If you decide to take it home, it is worth every penny of $550.
Here's my CKC.