Thanks for the input, very valuable. Now I know for sure that the option of building a full hull and then cutting that to size with a jigsaw or something similar is not a good idea :)
In the mean time I have already decided on a solution similar to what you are describing.
I will assemble the total center keel and cut it at the waterline, then glue all the separate pieces vertically onto a perfectly flat base plate of 3 mm plywood. I will then make waterline cuts on all the individual frames and glue them onto the base plate and center keel one by one. That 3 mm plywood base plate will also provide the extra needed height to make the model sit in the water at the correct level when it is completed.
Concerning the Minibrass PE-parts; I was already planning on buying those :) The only part I was doubting about is the B/C deck longitudinal profiles. The Amati full kit version of the model already comes with a nice amount of PE-parts including B/C deck longitudinal profiles, and they already look pretty good. I wonder if it is worth the extra 157 euros of getting the Minibrass versions. I have never seem them up close side by side. What do you think?