By now I found a very powerful and free program that is a definite overkill for my needs. Open Source, runs under windows and/or Linux, supports any file format I might want, has lots of tutorials in text form and on Youtube. Just if somebody happens to require such a program as well: I will go for Blender. Not exactly a CAD program but definitely sufficient to model some cases 
Meets all constraints but "easy to use". On the other side: by now I understand that no 3D program will be very easy to use for beginners and powerful at the same time. So I will invest the time.
Udo