I've just been through this whole episode of trying to sort out my suspension on my 2003 156 SW JTS.
Here's my guide to sorting out what you need and to check;
Look on the lower back of the rear struts for these numbers 60665438 or 60668186. If you have either of these you have nivomat, if not, lucky you all you need is standard dampers.
Assuming you have nivomat the car is approx 20mm lower than standard so you have two choices to retain ride height balance front and rear.
1) Replace springs and dampers front and rear with standard ride height springs and dampers of your choice.
2) Replace Springs with Eibach pro springs 30mm lower than standard, 10mm lower than nivomat cars and dampers of your choice.
The top mount of the nivomat struts can be reused.
Check all your wishbones, drop links etc for wear and get them done at the same time while its in bits.
The sportpack rear springs are for saloon only not for SW, they may be ok up to you if you want to chance it, but pro springs feel better than the sportpack fronts did.
If you drive over a lot of speed humps or bad roads check your undertray after the suspension has been changed, I fitted an aluminium tray to give better ground clearance.
All in all it isn't cheap to get sorted, i've spent almost £1000 on pro springs, Bilstein dampers upper and lower wishbones, drop links and an undertray.
There is no buying advice regarding nivomat fitted cars, it was a nasty shock when i realised what was needing done. Anyone looking at a SW needs to check whats been done as they seem to fail well before 100k miles. The nivomat struts on the back of mine were well fooked! One of them had no resistance at all, the other was seized! One of my friends has a SW and his rear end has given up too, so its certainly not uncommon.