Thank you very much, tpartner and holch! This works perfectly fine
I hope you don't mind if I put your answers into a step-by-step solution for future inexperienced users with the same problem (as I as a complete LimeSurvey-newbie had some initial troubles to understand what exactly you mean)?
1.) Create question of the type "equation", activate "always hide this question" in its "Display" settings
2.) Put text to be used in the notification into the field "Question" of the question
3.) Repeat steps 1-2 for every text-element that you want to use in your notification
4.) Reference the hidden question(s) in the notification email template using an expression script (i.e. question code in curly brackets). Here, you can also enter the conditions that define whether a certain text-element should be shown in the notification.
Example: {(if question1.NAOK == "Y", textelement.question)}, with 'question1' defining whether the text-element should be shown, 'textelement' being the text put into the "Question" field of the equation, and the suffix '.question' making sure that the HTML formatted text-element instead of the unformatted text is used in the notification.