I am proud to announce that the machine is working again

A local Amiga-techy took the motherboard for inspection and fixed it (faulty U106, details to follow).
I have just set the machine up and as I write this, it's humming on the background. The original SCSI hard drives were still properly set (SCSI Ids and terminations) and both work; there is even a complete Workbench 3.1 installation and a lot of software! Some of them in german, though.
I also installed an Indivision AGA Mk.II and the machine is now connected to a 22" LCD monitor (thrue DVI-cable).
I will post more photos and longer story here later.
This is truly an awesome machine! Thanks for all your ideas.