Thank you for answering :-)

But still, I hope you can clarify a couple of more things for me before I get started:

By basing my site on a demo, I guess that I will be doing much of this:
– deleting menu items
– sorting menu
– deleting pages

Are all these the type of changes that I can do in WP dashboard (and dont need to use custom css or edit function files to do)?

I have read that if i want to make changes to function files, I need to use a child theme. Naturally I dont really understand what a function file is yet :-) …but what are typical kind of changes that I would need to do in a function file?