Hello Tudor,
thank you very much for your answer. It helps me already to get a better understanding.
As for PPM 6.0 I see the same and follow your suggestion. We will focus on PPM 5.0.
Still I got many questions 
a)
Is it possible to run Resource Management without any backend, i.e. create ressources manually within PPM? The management likes to follow a step by step approach, so that integrating HCM in a later point in time. Does this causes any severe problems?
b)
Using Portfolio Management is not planned for now. I asume we can ignore the most of the config for Portfolio for now - right?
c)
You say "....Depending on your "company-specific" requirements" - can this be done by activating certain BADi?
Actually I think we will start with the given standard, but have no idea if this will be sufficient.
What were the main reasons in your project to perform changes in PPM? Why reinvent?
d)
We are using CATS - but have no PS running. Is this a requirement in order to integrate CATS with PPM?
e)
As for configuration and customizing I will follow the Master Guide, ConfigContPPM50SP11 and the SAP Notes.
Do you got any suggestions or recommendations for other documents or sources besides this and help.sap.com or SCN?
Kind regards
Darijo