dude you will get as many opinions as people on here......I'm not trying to be a di** but......
if you spend a couple hours using the search button you'll then have a very good idea of what products are out there and what things cost..... then maybe people on here can help you fine tune
Your gonna get that side to side movement.....Its called bumpsteer....They have kits to help aliviate that side to side movement and all over the road feeling...
Why tell him to get new springs??? go get a different car if you only want a smooth ride. the ride will be the same or only get stiffer even with ford c springs or eibach pro kit. (it will corner and handle better, but it WILL NOT be smoother)
your stock shocks are probably old and crappy also....you're not really going to get a smoother ride with any aftermarket springs unless you replace the shocks and struts. you should really think about investing in some quality shocks and struts, but even then, i can almost guarantee you wont be happy with the ride quality you're looking for.
FYI....the car will never ride much smoother than it does now. of course its going to be a more rocky ride than a jetta or celica...its a frickin mustang!! go buy a used benz or bmw if you want that luxo-boat slush box ride.