http://git.eclipse.org/c/e4/org.eclipse.e4.ui.git is the old Eclipse 4 code -- it has since graduated from the e4 incubator and been merged into the Platform UI code base. This repository is effectively obsolete.
I want to reuse the e4 Quick Search in my application.
First I have to check out the required code, I think it's the repo: eclipse.platform.ui.git
What target platform I have to use for this bundles?
Right -- you want the platform/eclipse.platform.ui.git repo. If you develop using 4.2, you won't need to worry about setting up a target platform for the next while as everything will be based on 4.2. Or put another way, your 4.2 installation is the target platform for now.