A real Nismo kit will have a sticker on the inside of each section. IIRC its not removable, so unless the inside has been painted then you will see it.
I also think that Nismo kits are fibreglass and not plastic, which some replicas are.
Glad you have found something that fits your requirement .... and a Vauxhall is indeed unexpected
I have always found Lexus dealerships very helpful and most of all professional, which a lot of other dealers could learn from.