Well, I'd love to have your "problems", Luigi.
The Murcielago might be an alternative, maybe you should truely take it in consideration. I don't know about the reputation of Lamborghini in Italy but since Audi overtook the company, quality has improved substantially. So if you're concerned about quality, you shouldn't.
Unfortunately there are not many "affordable" true supersportscars around these days.
If I had the money and gutts, I'd probably go for a Murcielago. You have to ask yourself what kind of car you really want. As far as I know, you already own a comfort oriented cars, the S55 AMG. This means that you can have a sportscar without compromises. So go for it. If you want a sportscar for the track, the GT2 is a very good choice but you have to have it modified by a pro. Reducing weight and optimizing the engine output / suspension setup is vital for GT2 track use. But I think you already learned that.
If you want a sportscar solely for street use, get something wild, flashy and fast. You'll never be young again and with 60, you can't afford driving a Murcielago or even a Modena because people will start talking behind your back, right ?! The choice is yours but I have to admit that it isn't easy, even if you have the money.
Personally, I love the 575 MM and the Murcielago but you shouldn't listen too me too much. Yes, the Maranello is based on an "old" design but actually the GT2 is based on an old car too.
I'd set myself a limit how much money I want to spend. This already excludes/includes certain cars. Then ask yourself for what purpose you want to buy this car.
Last but not least: if you buy a Modena, you'll have substantially less performance compared to your GT2. Are you sure you won't miss the GT2 ? Almost 4 (!) seconds from 0-200 kph mean worlds, not to talk about the handling of the GT2.
A last proposal: why don't you keep your GT2 and sell it as soon as the new Modena/575 shows up ?
The choice is yours and nobody can give you the perfect advise.