Hello and welcome @Ben_Liebrand 
Having never created a Sitely website with more than 30 pages, it’s difficult for me to answer your question… But for a site of that scale, I’m not sure Sitely is the most suitable choice—especially if you’ll need to update it regularly.
@maxim I’ve already answered some of your previous questions (see: I can have folders but I can't make folders). Folders and sections are not the same thing; it is indeed possible to manually create folders in Sitely. And yes, deleting a section will also delete the pages it contains.