a) When a guest user log in, then he need to be able to see the welcome (or news) screen. This will give him a overview of the project status as well as important information.

Just give access rights to Today screen for Guest profiles.

b) Some document they will need to get access too. I have a need to share with them only the published / approved documents

Easiest way is to define a specific project (or sub-project), affect users to this project, and set documents on this project.

c) Then for the various project roles, I want to define specifically for the project documents, limited access. this means that draft documents will only be accessible to the Project Management team, until it is final, then once it is approved, then it will be issued to the project steering committee. Once they approve, it will become "public" accessible

There is no visibility restriction defined as described. Only access restriction at project level can be defined.