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You might want to visit the GM/Chevrolet/Corvette web site and try some of their build configurations. You'll get a better idea of what the options are (including exterior color and interior seat).
I don't believe all C6's come with the NAV (screen). I think that's an option. I have the Bose Stereo/6 CD changer option and I think it looks just fine. But what looks fine is subjective.
The cheapest version of the coupe is the 1LT option. No choice for color of the interior - it's black, no matter what the color of the exterior. The cheapest version of the vert is 2LT - I think you can get any one of the 4 interior colors with most exteriors.
The screen is with the Navigation option, not the gps.
Can't answer your question about radios/ac. You haven't really defined "nice" for me. I haven't had any of the cars you say have a nice radio.
The best way to answer these questions is to go to several dealerships and check out their inventory and ask the salespersons your questions. I am sure the would only be too happy to talk to you.