Hi Enio,
First of all: congratulations with your achievement!
Maybe know you can become an active poster
During the development you can indeed with the search engine find an answer on all possible questions you have, because I think I've answered them all more than once
Regarding your GUI: I had a similar approach: a dialog to input the configuration settings and my main window was also kept clean. The main difference is my main gui's constructor receives the mode the application is running in, not the service returned by configuration settings dialog.
Kind regards,
Roel