I am interested in a DRM-free EPUB (Switching Social) available only on the Kobo store.
If I purchase it, can I download just the EPUB file, without having to use the Kobo app? I'm quite happy with my existing EPUB reader.
Yes. Once you've purchased the book, you can go to "My Books" and find the one you want to download. Below the thumbnail of the book cover there is a "..." menu which looks like this when expanded:
Click "Download" and you'll be presented with a dialog:
Now you can load the downloaded epub on any reader you like.