The whole strut & spring thing is hugely confusing
@DrWong . Especially if you want to keep it factory.
And it gets even worse with Sport parts.
You can’t get Sport struts/shocks apart from BMW. You can’t get Sport springs (-15mm) apart from BMW.
You won’t find part numbers online for springs. You have to give your VIN to the dealer. My favourite BMW parts counter recently humoured me and fed all 6 of my VINs through.
This identified 2.2 and 3.0 Sport springs are the same. Then there are 2 different springs, one for cars with air con and one without. Go figure that out! These were all manuals. Possibly automatics use different springs.
I’m currently trying to assemble data on what the correct lengths of spring and cross section of the coil are. Plus the correct colour code system the production line uses. You can see that on your spring. It’s a red/red. Which from my limited data currently suggests it may be a Sport spring! From memory a Sport spring is 33.5cm and a non-Sport is 35cm. But I’ll have to check that.
Many suppliers will quote standard struts and springs as the correct replacements for Sports.
For the struts/shocks that’s less of a concern as I’ve learned from experience. Something like a Monroe Reflex is superior to the original Sport part anyway.
But the springs are a different kettle of fish. A supplier will happily tell you standard height springs are correct for a Sport. They aren’t. And neither are Eibach/H&R at -30mm.
This causes a situation where if a single spring is replaced that corner of the car sits at the wrong height.
It’s less complicated with non-Sport. Although I have an array of non-Sport non-OEM springs and they can vary by up to 10mm across manufacturers anyway! But in theory that is I believe within BMW tolerance. But would you really want that? For that reason I would say always replace in pairs.
If I was doing a non-Sport suspension I’d have no problem using KYB if it saved me money against Sachs.
In fact I have an almost new pair of KYBs removed from a breaker I’m saving for my brother’s 1.9.
Which is a good point to remind you that 4 cylinder springs are different to 6 cylinder due to the extra weight.
You can trawl the internet for ever and a day. But you’re unlikely to find a comprehensive understanding across all Z3 types (or other BMWs) presented by anyone. It remains a big mystery. I doubt even BMW could tell you anymore. They’ll just supply the correct part off your VIN.
But apart from wanting to improve stance/handling that I guess is why some people say to heck with it and go for Eibachs at -30mm which puts everything on a level playing field!
And yes. Would be great if you could replace all flexible joints and bushes while you are in there. But watch the pennies and watch the time it takes.
Sachs Super Touring look like a good bet so reckon I'll try to get those now, along with some Sachs springs. 
