I’ve been running Sottozeros for the past decade (different sets, of course) on my C4S. Not sure what you mean by ancient construction if they work. Not knocking the Hankooks as I’ve never tried them, but what gains are you expecting from the Hankooks unless it’s simply a pricing issue (which is fine too)? With the advantage AWD, the Sottozeros work well until about 5" of snow, after which becomes a snowplow (but by then I take the SUV anyways).
That’s your imagination. A reputable independent dealer are just as good if not often better. When I bought mine, also preowned, I intentionally avoided the Porsche stealership, with zero regrets. But hey, if you’re willing to pay a premium, go for it.