I made good progress in the past days. Today I finished the basic AI for my workers and zombies. They now can detect each other and will fight until one dies. Besides that workers will look out for spawned organs. Once there is one a worker will move to the grave, pick up the organ and bring it to the sellbox. (But they will interrupt this at any time in case when they detect a zombie. Killing zombies has the highest priorty)
Some parts of this you can see in action here (the video is from yesterday and doesn't show the latest ai):