We prefer using English here, meanwhile.
As several posts stated, the best way to install UCS is just to skip the installation of additional software during the first install. Once installed, use the AppCenter to add the desired software packages.
Regarding the Owcloud Appliance and resizing have a look here. It is a little bit advanced, indeed. But doable.
Operating systems like UCS are a really complex kind of software. As you have noticed issues will happen. Univention tries to minimize these issues but we can not prevent all of them.
The point of getting a subscription is to have the possibility to get support regarding any question. Including remote hands where we will assist you. A subscription does not prevent you from having issues, but it helps to get over them. If you have issues with the Core edition it might be a good idea to have someone reliable helping you with issues.
Greetings
/CV