I'm no expert, but I must disagree with that statement. At this point, I believe there to be as many, if not more choices in 18 than 17.
I do agree though, check your backspace measurements many times over. I'm not doing a stock rebuild so I don't know what will fit, but when I was considering doing an RMS conversion up front, I was looking at 18x8 with the standard Mustang back-spacing.