If you get RPO1's with Kumho Ecsta Supra 712's 18" wheels and tires are very reasonable. You can get the 18x9" 42mm offset wheel with 255/35R18's front and 18x9.5" 45mm offset wheels with 275/35R18's for the rear. I would be more concerned with increasing the width of the front wheels before the rear. The car already understeers to much stock.
Later, Steve
Later, Steve