ebennz - yes, I changed both the horsehair and springs on passenger and driver seats. I didn't put in new leather, but that shouldn't be too much more of a problem.
The most difficult part of the job for me was to reinstall the seat motors/tracks. I made sure to only do one side at a time to have a template to work off of.
http://business.baylor.edu/Richard_Easley//autofaqs/seats.htm
That site will help give you an idea of what the job involves, but when I did the job I didn't really reference it.

You could also try to put tennis balls in your old seats to save on cost/time. Some members have tried this and said it works well. I tried it when I took my old springs out and while it helped a bit, it wasn't as good as the new springs.

Sorry, I can't remember the eBay guys name, but he was just selling because his car was totaled before he had a chance to install them.

good luck!

Peter
1988 420sel