To answer your question about will 18's fit vs 17's, the rim diameter is not a factor to fitment. The tires you put on will be though. What this is called is upsizing or downsizing. Whatever wheel diameter you go with, the tire will have to be sized accordingly by height to the stock tire. Just remember 3 things about tire size, for example, 315/35/17 (315 is the tread width in mm's, 35 is the tire sidewall height and 17 is the rim diameter) to put into perspective, 315mm wide is the tread width, 35 means the sidewall height is 35% of the tread width, which in this example is 315mm. I went with staggered 17's on mine as that is my of my liking, staggered meaning the wheels are of different width, as mine are 17x9 & 17x10.5 and the front tires are 275/40/17 and the back are 315/35/17. Now even though the tires are of different size, the overall diameter is the same of that of stock setup. If you go with 18's, and you are looking at 18x9's for example, you would want to go with 265/35/18's. If you wanted to go staggered as I did but in 18's instead of 17's then you would go with 18x9 & 18x10 with tires being 265/35/18 and 285/35/18. Those sizes mentioned are the same overall diameter of stock tire height and will not offset your speedo.