For the most part, 225/40/18 tires are considerably cheaper and more common than the stock 215/45/18. Stock wheels and suspension, you'd have no problems with 225 tires. I considered 235/40/18's before I traded for newer tires.
That's exactly where I'm at man. I took my '06 MS6 GT in to a Ford dealer and they offered my $12k for it. The guy said that's the average auction price; and that he had seen 63 of them at last month's auction. I would'vt called BS, but I almost broke the hood latch on the Focus we were looking...