Hi emi.b, 18's fit on the GD and have the best look imo, you do lose a little ride comfort though and the smaller profile tyre will also transfer more road/tyre noise into the cabin so be aware of that if you go with 18's. 18 x 8 rim will fit easily with a 225 x 40 x 18" tyre
17's might sacrifice a little for the look but gain points for ride comfort, all depends what you're after 
Agree totally with CraigB's assessment.
My car has the oem 18" wheels with oem Hankook Ventus Prime 225/40R18 tyres. I like the look of the oem 18" wheels on the car and the oem Hankook tyres are, imo, satisfactory for normal suburban driving in terms of handling in wet or dry.
But, I seriously dislike the tyre's low and stiff "40" aspect ratio required for the 18" wheels. On even, nicely surfaced roads the cabin noise is acceptable but on coarse/open road surfaces I find the cabin noise too intrusive and annoying...so much so that I intend to go 17" wheels with 225/
45R17 tyres. That should give me the same rolling diameter (so will not affect the speedometer) and will fill up the wheel well pretty much the same.
It shouldn't be too hard for me to find a 17" quality built light-ish wheel that will look nice on the car. However, finding a very good handling tyre that can take the occasional track practice or driver training day AND is noticeably more
quiet-ish than the Hankook's will prove more difficult and requires a lot more research on my part.
(I accept that my noise issue with the 18" tyres is not helped by being a 76-year-old grumpy fussy Virgo with hearing problems in the left ear...the ear that passengers talk into over the radio, backseat conversations and crappy road surfaces.
)
