Welcome. As far as modding the car, you'll only void what you modify. As far as springs, both Eibach and H&R are good companies. I have the H&R sports, and they are great. I rode with them on stock shocks for 2 years, recently upgraded to Koni adjustables.

With the Sportline springs, you'll likely want camber correction and possibly new shocks right off the bat due to the larger drop.