After starting all over with the head model mostly knowing what I was supposed to do, I am satisfied with the outcome. It was hard work to keep an eye on not making any meshmistakes and creating a proper edgeflow.
I still have to practise a lot, but for my first try I think it turned out alright.
This project made me realize how important it is to know the anatomy properly whenever I sculpt something in 3D. This basic knowledge is necessary to create polygons that can be easily animated later on to create a realistic and natural movement. It was really useful to get a hang of working with Mudbox as well.