OP, you need to consider a few things when going wider/bigger on your ride:
You are only going to properly fit a 245 max on your current wheels, maybe a 255 if you go high enough profle, I haven't researched it. You are best to stay about 25"~26" overall diameter, so that is going to create its own limitations.
The 235/50/18" tire = 27.3" diameter and a bit larger than I would like to see on your ride, especially if lowered. Now if you had a 16" wheel, that size would be no problem. You will also effectively raise your final drive gearing going larger diameter as the tire will make less revolutions per mile. This is not a huge issue, but part of the equasion as you will lose a bit of giddy-up off the line. You will also elevate your car going larger tire than you currently have.
What are your goals here?
Jazzer
