Yes, you're in the driver's seat but don't get too comfy there.
One owner, good vibe from owner, all records, 10/10, low mileage cars are not easy to find. You are also not the only one who knows that convertibles are better bought in winter, especially couple of months before spring. If I was in your shoes, and I liked the car, I would try to bluff-haggle a bit but wouldn't blink paying full price especially if it was fairly priced to begin with. $0.5-1k is chump change when you're buying a Porsche, given the high maintenance costs. And you're going to blow that $ on needless 'mods' later, anyway.
So keep things in perspective, get the car if it passes muster, and sleep tight even if you know you could've potentially paid $500 less.
Z.