Welcome to Stangnet. My experience is 99% autocross. There are some important differences when building a car for open track events. There are some tips that will work in both situations:
1. Get the chassis as stiff as you can. Every input you make goes through the chassis. Stiffening up the chassis will help you get the most out of the other mods you make.
2. Add adjustability to the
suspension. Being able to adjust the shocks & struts (and rear sway bar, if you can find an adjustable one) allows you to tailor the car to your driving style.
3. Always ask yourself, "What am I asking the car to do that it can't do?" before making any mods. Many people make mods that they don't need.