Unfortunately I'd agree with the garage. If I was buying a Ferrari from a main dealer I'd want them to be highly professional on every level right down to the coffee they serve . Their service needs to be appropriate for the amount of money I'm handing over.
From the perspective, selling cars has to appeal to the buyer's emotions, every buyer is told the car is perfect for them, we got it in just for someone like you, it's a rare model, you're lucky to have a chance to buy this one etc. The buyer is in the mindset that it is their personal car before they test drive it. The danger is that seeing some kid playing with the car and getting his greasy fingers on the shiny gear shift shatters that illusion.
I think you'd be best going for a car club, that's an environment where the owners want to share their pride and joy with everyone.
By the way I'm finding the opening line of "Welcome to the 458" with a 458 logo gives the impression that you are reviewing at a 458, but then you say "I'm going to show you a BMW". It's a bit muddled.
Oh, and be careful not to make comments on public forums about people you want to do business with, what if they read what you said about them?