A bit late getting on board here.... Do not buy/use used SE rr springs. Although they were the stiffest factory offering, they had problems with premature breakage (which I experienced first-hand).

Also, changing the rr sway bar, straps & bushings will provide the biggest "bang for the buck" in terms of handling improvements. However, if you go any larger than an early SE (19mm) you will need to reinforce the subframe which is not likely to be cost effective unless you've got a welder in your garage. Even a 19mm rr sway bar combined with a well worn subframe can lead to damage/failure of the subframe. Yes I've been there/done that too!

Hope it helps.

Redcoat Raceworks. Performance parts and custom fabricating for Contours, Mystiques and Cougars. Specializing in chassis and suspension parts. Custom end links, control arms, strut tower bars, engine torque braces, etc.