Whether you want 17's or 18's is up to you, and depends on what you want to do with the car, how low you're going, etc. Spend some time looking through the member rides, and decide if you like the look of having more tire/less rim, or less tire/more rim.
If the only sizes available for 17's are 17x7/9, you'd either need to basically slam the car and do some fender work to make 17x9's look good (since they will likely poke if you're not low enough), or put on the 17x7's and be good to go...but that's pretty narrow. Either way you don't want to have a 7" width rim in the front, and a 9" width rim in the rear. You need to find out all the offsets for these wheels btw.
If it were me, I'd go with 18x8.5 all around. But then again, I wouldn't be going with XXR's. But the good thing about a lot of people having XXR's is you can see the rims you want on a lot of cars on the site, so spend some time looking as I mentioned, and see if you like 17's or 18's better.