Switch IT to act as a separate company

Having IT dept working for 4 companies within the same group raise questions of how to allocate IT expenses, from which company take the budget….
after a negotiations with the group management it was agreed on transferring the IT dept to act as a separate company giving service to all companies, it was requested to prepare a proposal for the new IT company; the proposal should include:
Organization chart
Job description
Scope of work
Salary scale…
If you can help in this; put your ideas to be discussed