As I am writing another computer is retreiving data from the failed harddisk.
After 3 weeks of searching and reading forums I finally found some affordable software to repair the firmware of my WD1200JB-00CRA1. No new PCB, no solding, only a scan with MHDD which gave all IDNF errors.
Thanks for the link.

I mean, after the scan I did a scan with A-FF Repair Station (http://www.hdd-tools.com) which concluded that a repair of the firmware on the SA was possible. Bought one license and was finaly able to rescue the data.
By the way it still gives S.M.A.R.T errors but who cares aslong as my precious data is save again.