Sounds like you have money that I don't have
I will just confirm the bit on the Nissans being heavy. I've driven two 300zx's, both with 6cyl non-turbo engines. One was an 86 automatic the other was an 89(i think) 5spd manual. Both cars were very fun and were pretty quick(I bet a turbo in it would make it fly), but I could DEFINATELY tell that they were just heavy to drive, heavy steering, etc. See I've got a Volvo 240 DL Wagon and it's just an amazing handling car, don't knock it til you drive it. It's got the best steering radius of any car I've ever driven. Even my dad who is a 20+yr experienced mechanic says that he hasn't seen anything like it before.
Erm, anyways..... Back on topic. I wouldn't think that the style of the 300's has changed much either. I know that in your list your talking about the newer models but I don't believe they are much different from the older ones.
I pretty much dislike all mustangs, they really aren't that powerful, just a sort of "American Icon" on sorts. I have no idea what the 3000GT is like, so I can't help you there.
If you want something to make lots of power, etc. then I would reccomend getting a Camaro or a Trans Am. Personally I like the looks of the Camaro better, and Z28 or SS doesn't matter to me
As for the RX7 I asked me dad about them and he said they have incredible horsepower potential but are pretty horrible on gas...
So really it depends on what your looking for. You have V6 and V8's listed, the V8's(any of them), won't be that great on gas, and a turbo charged v6 wont be much better. So you really need to look at what kind of drivng your going to be doing and what you want the car to do before you can make a decision.
I can spend your money for you
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btw... I have an 83 Z28 camaro for sale... BUY IT NOW - You could buy the Volvo too... hmmm