you can lower your car with just the springs. But it is better to get caster chamber plates, bumpsteer, and new polyeruthene(spelling) bushing. But you can put just lowing springs on your car. If you don't put in chaster chamber plates then the inside of your tire will start to wear worse the any other spot.
Unless you drop the car more than 1.5" you shouldn't need C/C plates. One thing you will need are shorter sway bar end links so the bar works properly. I wouldn't worry about C/C's unless the front end align after the springs settle in.
all you need is the springs you want, if you go more than 1.4 inches get CC plates to correct the alignment (make it better than stock) i would get some regardless even though they are not needed because with stock adjustmetn and 275 tires my tires wear on the inside..... and with cc plates you can get it aligned and make them wear correctly.
you do not need new struts, shocks, bumpsteer kit, anything like that its good to have them but I have ONLY springs and after a year no problems.
If you want to improve your ride quality and handeling over all, then yes, you would do things like upgrading sway bars, shock absorbers, struts, etc. However, if you're just looking for a much improved handeling car, and also for a "lowered" car (way better looks), then yes, all you really need is springs that will lower no more than 1.5 inches.
The only time I have heard that you need a bumpsteer kit is when you use the steeda special ball joints, then they say you definitely need the bumpsteer kit, I say stay with stock ball joints and you won't need the bumpsteer kit, I have J&M 4 bolt caster/camber kit, Cobra bilstein struts and shocks, shortened end links, cobra sway bar, 03 Cobra IRS unit, Eibach Cobra R front spings, Steeda Cobra rear springs for the IRS, with MM mounting bushings, and MM aluminum rack bushings, with my Cobra front brakes and cross drilled and slotted 13" rotors, now if UPS will just bring me my SS Steeda brake lines I can put them on, bleed my brakes and ride again, LOL.