First,
welcome to the board! I have most, if not all, bolt-on modifications on my Z. They are listed below in my signature. However, there are several different brands you can go with. I would suggest checking out the Tech Area threads. Hope this helps at least a little bit and puts you on the right path.
And blue is right....you probably dont want the Nismo if you plan on modifying.
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SpinTech Mufflers, Airaid CAI, JBA H-Pipe, Lethal Performance Over Axle Pipes,
MMD Quarter Panel Louvers, Cervini's Ram Air Type IV Hood, Carbon Fiber Radiator Cover, GT500 Rear Valance, Quad Exhaust Tips