You may also want to add a front strut brace to help reinforce the top mounts as they aren't all that on the z4 and may mushroom or crack. It will also stiffen the front end up a bit. You can get reinforcement plates that sit under the shock tower but they are like gold dust and will raise the car up a bit at the front.
They aren't crucial as the eibach aren't meant to be much stiffer than the standard springs but I'm looking to invest as a precaution.
If you do go for a strut brace get the wheel alignment done after this goes on.