FULL body kit? Front/rear/sides for only 300? Was it used and what color is it? If your not going to buy it, you think I could have it? Anyways, fiberglass kits vary in quality and even the most durable fiberglass kits will completly shatter if you get into a fenderbender or scuff your kit. Fiberglass in a way is good, because it is very inexpensive to produce and can be easily replicated and modified by people. All in all, it depends on who you're buying it from.
I recommend you spend your money on polyurethane because if you bump into something, you would only have to repaint the kit. But if its cracked, there's really no way to fix it, except buying a new one, unlike fiberglass. That's why there's so many ricers and wannabe racers out there with cracked bumpers.
As far as my best suggestion, get fiberglass for the rear and sides but get polyurethane (or abs) for the front. As a matter of fact, also get a front lip since not only does it look cool but protects it as well. But if you're a true racing fanantic or just plain hardcore, get the carbon fiber.
Hope it helps