With the hood situation, Cervini's does make excellent quality hoods, but it is always recommened to get hoods pins just in case.
And with any bumpers or side skirts, you will want urethane over fiberglass. Fiberglass bumpers are very poor quality and most shatter from one hit parking or a rock on the interstate.
Here is a few samples of what is out there for the '99-'04 Mustang.
Roush Stage 3 Body Kit
Roush Stage 2 Ground Effects
Saleen Body Kit
Cervini's Stalker Body Kit
And here is some of the most OEM products.
The '99 Cobra with Mach 1 chin spoiler.
The '03 Cobra front.
Hope that helps you out some.
