Re: When to change suspension?
Its usually the shock absorber part of any suspension system that needs replacing. If I am not mistaken, on the average under normal driving conditions, OEM shocks lasts approx. 50k km before they go.
If you are looking at maintaining your OEM ride height and comfort, just replace your shocks with Sach/Boge, they are standard OEM parts.
If you want an aftermarket non-lowered setup, you can go for Bilstein HD or Bilstein B6. These are shocks alone to be paired up with your OEM springs, hence no lowering. The ride will be firmer no doubt, but nothing that is considered uncomfortable.
Koni FSD can be used with non-lowered OEM springs, although few people here have paired them up with Eibach or Koni lowered springs (35 to 40mm drop). IMO the ride is definately still comfortable for an aftermarket setup.
If you will consider a moderate drop, there should be some options out there for springs + shocks set that will lower your car 20 to 30mm without sacrificing too much comfort.