So my Luxemotor will be just under 25m. For this size I only need to have a "small" license.

That may be so for European waterways. But if I have understood correctly, the ICC certificate proves only general competence - and you may still be barred by local restrictions. And that may be less than 25 metres. Here the limit would be 15 metres (and you would also need the ICC costal licence).

The idea about the pontoons is a good one, and it's a cheap one.

There is no such thing as a cheap boat [:)]

You are right that the pontoon solution will not let you exploit volume below the waterline for habitation, and you will loose headroom. Still, I thought most European waterways had air draft of > 3 metres (?) - and that should be sufficient for a pontoon boat?

ok