There are many body parts and accessories to choose from for enhancing the look of your car. For the most part, side skirts and rear bumpers on a street car are visual upgrades and will not affect handling. At high speeds, the front bumper will affect handling. Read my accompanying article on aerodynamics “Optimizing the Aerodynamic Balance of your Car” for advice on setting up a car to be aerodynamically balanced.
If you do not plan to race your car, then a body kit is a purely visual upgrade. But for racing, I recommend installing a lightweight front bumper (preferably one that has been aerodynamically tested).
Body kits are usually made from fiberglass, ABS plastic, polyurethane, or carbon fiber. Carbon fiber is the best material to use (at least for racing) because it is lightweight and strong. However, it is very expensive and cannot be repaired if it ever breaks. ABS plastic and fiberglass are also light and not even close to as expensive as carbon fiber, but since they are rigid materials, they tend to crack easily.
For street use, durability is the biggest concern. Weight is not an issue. Therefore, polyurethane and ABS plastic are good choices. They are both flexible and can take a lot of abuse. Read the Body Kit Product Guide on sportcompactonly.com for a comparison of the different materials and a listing of manufacturers that work with them.
The best option for installing a body kit is to take it to an experienced body shop. Even though the kits are made specifically for each car, there is a lot of prep work that must be done before they can be installed. It is necessary to remove stock body parts, drill mounting holes in the kit, and paint the kit before it is installed. Unless you have the proper tools and facilities to do all of this, pay a body shop to do the work.
sportcompactonly.com has an excellent section with information on choosing and ordering body kits. Read their articles on body kit selection, shipping, and installation before you purchase a kit.
Visit my website for more information about body kits and accessories.