Stefan, I have a Dufour 40 from 2003 that also needed upgrades to electronics. I was advised to install a network to link all the new components and it works well. With a GPS plotter you will have one speed source, and could possibly eliminate the moving paddle wheel speedo, if I was doing it again that’s what I would do. You will still need depth sounder which you would probably want to integrate to the network and all displays.
The membership association for owners of Dufour Yachts