I am from Germany as well.
(I can contact you directly if ya want)
Pocket Informant is the PPC app of choice when it comes to GTD.
(and you can fiddle with hundreds of settings for hours on end)
I use it to sync with outlook and have set it up by the "projects as contacts" method.
So each project is a contact (categorized under "Projekte" and "Someday" )
that can link to various tasks.
I use the contact notes to outline the project and the tasks needed. The first one gets to be a NA. When the NA is done, the project gets updated (I delete the first NA from the Note and make the next one an NA).
For an in-depth explanation of this method, look here: