Problem with the 5 speed is its weak, 350bhp and if you don't drive it with some mechanical sympathy it will shat itself. Can get uprated 5 speeds (PPG, Albins etc) but they are typically 3k + so more than the cost of a 6 speed conversion.
17's will handle and ride better than 18's. Standard caliper is pretty weedy so your best bet is fit a 330mm kit which fit underneath 17's, D2 or similar is a nice cheap upgrade or 330mm 4 or 6 pot Ap's are the daddy. Brembo of an sti are a funny offset so wont fit under alot of wheels. Also braided lines and decent fluid.