We actually have done wide body kits before. One of my personal favorites was the Mercedes CLK 430 that we built. We don't make them anymore because they are just too expensive to do it right. Mold cost for urethane is very high. For example, the front fascia that we've made for the 370...We probably have around 30K invested into the fascia...Just developing the molds.
Urethane would work well for the fascia and rockers but it won't work very well for the fenders. Unfortunately poly urethane doesn't have the support structure to work well for fenders without some serious additional weight to build support ribs. Fiberglass would be less expensive but obviously heavier and not as durable. CF would be my preferred choice but it won't be cheap. Although you'll be able to save some money with the hood. I am probably going to develop a pre-preg CF hood soon that is similar to what you have designed but we've moved the vents further back to aid in cooling.
The reason I mention seperating the intercoolers on the side is because that is how all of the professional race cars run it. That way the freshest air is going to the intercoolers and the radiator seperately. Brakes really just need air blowing across them so I wouldn't put too much emphasis on them. Something simple with a 2-2.5 inch opening will be sufficient. Check out this car built by Jun. I really like what they did with their intercooler/fascia setup...just throwing out ideas...
Nissan 350z R - AWD Import & Tuner Car - Super Street
Nissan 350Z Awd Jun Auto Vq35de Engine Intercooler Radiator Photo