Wow, that's some first post!
First off, fair play for getting one whilst so young, that's proper ace.
I think you have paid too much for a pre-facelift '06, hopefully you got one before the tax increase at least! Definitely not a Nismo exhaust, and whilst you can post links to the vid if you want there's no point as there's no way we can identify it from noise. Get some more pics of the exhaust itself, of any markings etc.
Bin the runflats ASAP, they're a stupid idea even on boggo BMWs much less a sports coupe. You'll need to replace both struts at the rear, doing one side only can cause some serious handling issues.