Um, with that intended user base you probably shouldn't worry too much about security.
Converting anything into ePub should be done manually. I can tell you from personal experience that converters do not work very well. Of course, some formats convert a bit better (open formats usually, like HTML/XHTML, ODT, tagged PDF, etc), but some manual layout and/or TOC adjustments will always be required.
- InDesign can export to ePub since CS4 or something like that.
- Word files can be saved as HTML, but you need to manually clean it up with batch replacing (to get rid of "mso" tags), then put through Sigil, which can check for syntax errors.
- For PDF files, you're probably better off OCR-ing them, unless they were saved as tagged PDFs. This format is intended as an output format, as a final destination.
Either way, don't expect them to covert 100% accurately; maybe 80% at best, but it ruins the whole reading experience if they are not checked and adjusted manually.