Could I just buy springs such as the Tein S Tech and add them to the stock suspension? Would it improve ride quality whatsoever or is it just to lower the car? Lastly, how hard is it to change the springs out if possible?

 

Of course you can. There are hundreds of Z's running aftermarket springs on their stock shocks (myself included). I chose the Eibach's becuase of their quality and the amount they lower the car.

As for improving ride quality, depends on the springs. The Eibach run just about like stock.

If you've never installed suspension on a car before, have a pro do it.

 

Any special tools needed to install springs?

 

A spring compressor is wise for the front. If you don't have one, then lay the shock/spring assembly on its side (aiming away from anything, and perferably on grass) and remove the top nut (preferrable with an impact gun). The spring and shock with shoot away from you a couple feet. Other than that, no special tools are required at all.