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Also, it is not clear to me you are going to run 18" wheels but the early Boxsters were not sanctioned by the factory to run 18" (or bigger) wheels. The later models were but they received some strengthening of some parts of the chassis to better withstand the forces 18" wheels can impart to the suspension attachment points.
My 99 Boxster came with option code 413 - 18" 993 Turbo look wheels. So 18's were a factory option that year.