After searching for a while i found Jomres. I created a test site, and after figuring out how the software works, which is not easy, I came to the conclusion that 90% of what I needed was already there, because of the tremendous flexibility. And for some other things the programmer created some adaptations. Some of the more complex things that I needed can be created using plugins. This is a very smart feature of Jomres. You can create your own plugins, just for your purpose. And if a new version of Jomres comes out, your custom created code will be maintained.
The current Version of Jomres makes it a lot easier for starting user. The support for Jomres is great. This is a very important argument for me, looking at my previous experiences.
In short: A Great piece of software for a great price. It can be used for rental agencies (builtin commisions) or property owners.