Hello Bernd.
My friend uses a Garmin chart plotter and despite being regularly 'updated' was in fact not as accurate as it should be and he was disappointed in its performance.
I use sea charts for the salty stuff and get them updated at a chart agent just before leaving.
As well as marking secondary ports I also mark reciprocal course/bearing, ports.
With these I use a simple Garmin 72 GPS, then if for any reason there's an electronic breakdown I can always navigate to a safe port.
I may sound old fashioned, I do embrace practical electronics but not gizmo's.
Pilot books are written with practical experience and give a better reflection.
You may wish to look here on TF for my posts 'Ireland to the Mediterranean Part 1' and Windmills and Wine' to help and give you idea's.
Safe cruising.