1. If you're new to the racing scene, the difference in wheel sizes isn't going to make a dramatic difference for you. You would be better off saving the 2k you're looking to spend and instead put it towards doing a track day or HPDE event. That way you understand the art of race driving and can better judge exactly what wheels you want. The Stock Rays will more than suffice for the learning experience.
2. The Diameter of the wheel (18" or 19") doesn't effect tire width, that's the width of the wheel. And most common racing wheels that come in 18/19" will carry the same widths of 9-11" so you'll be able to use the same width tires for either.
