Look into forums on what the who's who of car customization
Car customization is a tough business. If you will look at the number of people with cars that looked worse because of customizations, you need to know the who's who in the area that can help you.
Know the reputation of the company
It is imperative to know if the company is really aware on how to perform the customization.
Know what should be changed
You need to know what you can change and what you just can't on your car.
Invest in a good engine
No matter how you change the exterior of the car, you need to have a good engine in order to have the best performance. This is something that most people can't have with old cars. If it is a vintage car, it is imperative to know the condition and wear and tear of the vehicle.
Know why you are getting a customization work
There are some instances when the custom work is really unnecessary. You want to know why exactly you are getting one. Are you doing this to show off?