Sid,
Springs are one of those personal things, depending on how you drive, what the roads are like, and how heavy you (or your passenger 😳) are. I swapped out my original (16 year old) rear springs with the Spax dampers earlier this year, and installed AVO's with the 125lb springs. The car feels a little skittish at the back sometimes, and I can't get the power down quite as early as I could, so I'm thinking of dropping the spring rating slightly, but, there are other things to do as well - flat flooring, ride hight etc, so I have plenty to play with .
Jilles,
On the live axle, the rear bushes, where the A-frame joins the centre of the Axle are a disposable item, and will need changing around every 3,000 miles. This can make a big difference to the feel of the car. As you said, if your dampers are 25years old, it's probably time to retire them or rebuild them. Sorry, I don't know anyone who can do Armstrongs, but if they are original to the Seven I'm sure Redline would know.
Dave H