Thanks for all the info and input. I've done a rough calculation and the Nauticam with WWL-C, CMC-1, adapter and vacuum valve comes to around €4,100. The Isotta with strobe trigger (manual), macro port, adapter for the Sealux, port extension and vacuum system comes to around €3,650. To start with I'd stick with the 12-24mm and upgrade later, the 8-15mm is already looking very interesting and offers even more possibilities. Plus the benefit of a viewfinder. So it looks Isotta is the way to go...