I have no idea how to start with this. Yes ok multi-company will be the magic word. The companies are not related, they do not share anything. Only the user(s) are the same. They can make use of the same modules and the same localisation.
The reason I want to do this is saving costs for OPW and easier communication via OpenERP for the users.
I also would like to know multi-company technical basic architecture, how it is realized. This is to get more basic idea's about the issues that can arise and how to solve.