This question is also releated to the license.html of the
Flex2 SDK which is hard to understand. Especially if English is not
the native language. So my question regarding the use of Flex2 SDK
is: Is it possible to use the mxmlc for commercial projects or not?
If not, do I have to buy the Flex Builder for a license or is there
an option to buy a license only to work with the compiler?
Yes, you can use mxmlc to create a commercial application.
The thing that you can't do is re-ship mxmlc with your app, you can
only send the compiled SWF (and optionally your source code if
that's what your app requires).