How can I set the following as default for a document?
Body text: 10,5
Spacing between lines:
Leading: 3 pkt
Make sureyou have no documents open and then edit the Basic Paragraph style with this settings.
Just so it's clear, if you set defaults with no documents open, they will become the defaults for all new documents going forward.
To set defaults for just one document, do it with that document open.
More specifically to your desired list of defaults, you'll need multiple paragraph styles to apply different settings for headings, body, footnotes, etc. When you type text, InDesign doesn't know your intent; you have to apply paragraph styles in accordance with your wishes.