Buying a Nismo

Nismo450Z

I currently own a base model 370z and I plan on trading it for a brand new white Nismo 370Z that is on its way from the factory. My question is there anything I should be aware of before buying.

Information about my current z:
It's completely stock, I have owed it since October of 2010 but I bought it used yes its a 2009, and I owe about $23,000, there's some small interior damage on the door, and it's been in two fender benders that involved just a repaint.

The Nismo
It's all white, at the price of $45,950 I talked them down to 40,000, but could I got lower? And what's too low? And now that nissan puts the Bose in the Nismo could I bargain that with them as well to be included for the $40k or less? Could I make them give me more for my car if I didn't like the trade price? And last if it help I plan on putting $5,000 as a down payment.