Welcome aboard.
The genuine Nismo 350Z was only available in Japan & America with AFAIK only ever one being imported into the UK, ...which was apparently done by a Nissan dealership.
Some more info on the 350's can be found here: https://en.wikipedia.org/wiki/Nissan_350Z
You can of course pay extra if you like for a "so called" Nismo 350Z because of all the extra parts fitted (stickers, badges, skirts etc) but just be aware that it won't be a genuine Nismo.
If you are so inclined you could always just pay less for a decent 350Z GT and then add the extra mods you want afterwards which is what I'd do.
If Im right, nismo in the uk is just the body kit and nothing more. Mine was factory fitted from new but it could be retro fitted afterwards. Genuine Nismo kits don't come up often and will cost a lot so nowadays it would be cheaper I suspect to buy a Nismo kitted car than to buy kit at a later date. US Nismo cars have more mods but have you seen their rear bumper, absolutely horrific!