RE: invoice customisation
You need to have developer license to do any modification to tables. Demo license only allow you to do very minor modifications to few selected tables.
My suggestion is to contact a vendor who has the resources to do these required changes. Setting up NAV is very tricky, because one mistake in setups can cause many issues for the transaction entries and you will feel fed up of the system.
If you really think you want to go ahead alone please check the training meterials and videos related to NAV implementations.
Check this to download related PDF files :
www.cbsi-corp.com/.../07