350-ed

Hello, my name is Ed, and I am looking to buy my fist 350z. Love the car, I have test driven two last weekend and now I am hooked!!! Looking to get one in next couple of months, any ideas on what to look out for, any common probs I need to be aware of?? I test drove two gt's thought they were 280 bhp but one felt more powerful, but they were the same year?? Is there anyway that I can find out which engine is which, any codes on the engines?? Help would be great cheers ed