Any Mercedes will cost a lot to own and even more to tune- this is why I ditched them for my Z. Plus the automatic just doesn't cut it. Also a total piece from a driver's perspective.
The G35 is a great car but the rear seats are tiny, you might as well go for a 350Z.
The 350Z... it's heavy, doesn't really feel fast (weird), the brakes suck, and so does the interior. Despite all that it's a great car, especially to drive. The shifter is lovely, once you get used to it (it can be vague in the sense I sometimes wonder what gear I just put the car in with the only way to find out releasing the clutch) and the steering is GREAT. It's fairly heavy, but it makes driving fantastic.
The GTO is a great car. If you don't mind the heft, go for it! The V8 soundtrack is to die for, especially if you can go for an LS2 model.
The R32 is a great car too, if you can get used to the looks. Rear seats are very, very, very small.
If you can afford it, go for an Audi S4. If something goes wrong, it'll cost ya, but if it doesn't, you have a fantastic car. Very quick, easy to tune (if you get the TTV6) and a great interior.