I finally managed to upload Johann's modified Marlin firmware from my iMac onto the Sanguinololu (Atmega 1284P). Apparently the Johann-Marlin/Sanguinololu/iMac combo is not that wide spread yet. So it took me a while to find all the required info, but with some great help from Roland - who emailed me some very useful tips tips - I finally managed.
All the electrical/electronic parts are now connected and laid out on my desk, and I can 'virtually' print stuff using Johann's firmware.
One strange observation though: after 'homing' (I have to click the end stops manually as there is no real printer yet), and starting to 'print', the X-motor keeps spinning and there is no way, except resetting the printer in Pronterface, to stop it. Strange things indeed... but just another hurdle to take.