lol
all four of course because it affects launching and such. not sure hwat to tell u bout springs but i have H&R race and they are supposedly the stiffest compared to tein and eibach.
you dyno tune (when ur done wit performance mods) to get optimum air/fuel ratio throughout the rpms (i think).
summitracing seems a good place to go if you can pik em up and not pay ship.