I got many Mail with inputs how to solve my problems - thank you very much for support!
So the first thing I'm gonna do is to build some level-converters as on this page:
http://www.rocketnumbernine.com/2009/04/10/5v-33v-bidirectional-level-converter/
First I will try to get the SID-Socket running with a real SID and BiDir (the current solution is "write only" to the SID).