Get one that is still in warranty and put the extra three year factory warranty on it - $1300 or so, and good for up to 150,000km on the clock.

Then put a free-flow exhaust on it, and have it tuned for more power. A bigger intercooler with larger scoop would be a good idea too considering the weight you will be adding to it.

A chip really gets them moving once you open the exhaust up, but you should remove it (and all wiring) for services, Nissan may not say anything to you at the time but you can bet they'll note it on the service history and if you have a powertrain warranty claim you could have a problem.