AJAX? topic related formular inputs

Hi all,

I’d like to create a form with a selectable topic. How do I change fields for every topic instantly on selection (AJAX?) in best practice?
And how do I send them via E-Mail cutomized? I think about the topic in email-subject. Do I have to create a chunk for every selection if I don’t want to have all available fields in my email?

Thank you very much!
Smooth-Graphics