welcome to werkz
Nice ride - If you want to change your wheels, my advice would be, dont go bigger than 18 x 8J for the front, and dont go bigger than 18 x 8.5J for the rear.
I say this because your car is not the ZHP or "M Sport" model, so the wishbone ball joints and other suspension components may not be able to cope with wheels bigger than 18".
Also, if you want to go any bigger than 18", and want to keep the ride comfort, the total diameter of the wheel and tire will increase, and you will therefore have to get your speedometer re-calibrated, to ensure that it shows your actual speed.
Edit: If you do want to have a bigger a offset than 8J for the front or 8.5J for the back, then you will need to get spacers, and longer lug nuts so that you can push the wheels out to stop them from rubbing against the inside of the wheel wells