I agree with Hanno, dropping the subframe is a big, big job. By comparison spring replacement is about 2% of the total job - I'd keep your springs and see how things look. It's certainly possible a collapsed subframe bushing could cause the car to sag on one side a little bit.
One thing you want to check before you go too far with this plan is the bolts that hold the subframe to the unibody. They are usually very tight and 25 years of living in a fairly corrosive environment doesn't help. If you snap one of these bolts off you are going to have a much bigger job on your hands.
Here are some pictures from Dave's website that show various stages of the process.
http://www.w124performance.com/images/W124_subframe/