After a few weeks of planning, I decided to use the Fuji A303 as a basis for my digital stereo rig. One reason was that these cameras are readily available on Ebay for around 30-35 €. So even if this project would be a total failure, the risk is limited to around 80-90 €. Another reason was, that the lens of this camera is very close to the left edge of the case, so building a rig with one camera upside down would result in a stereo-base of only 4-5 cm.
To give you an idea what the final result looks like, here is a picture of the rig (although before the wiring was completed):