ok well back to OPs needs.
seems she is making worst possibel for choice for 2 reasons.
1. evidence to-date suggests azw3 takes more space than same book in epub
2. of all the popular makes - kindle is the ony one that does NOT accept additional storage via SD card
http://www.the-ebook-reader.com/ebook-reader-table.html
So, if the the criteria are
1. scope for lots & lots of books. ..
2. stored in the most compressed format...
then Kindle is fail + fail
or have I missed something ?
she already owns the books( in the "wrong" format for PW) so Kindle + amazon server backup/ syncing is irrelevant.
Get a Sony T2 / T3 & slot in a micro SD card , then sideload all of the epubs - job done - 8 weeks battery life...
or a Nook, or a Kobo...
& a free dropbox account - for a full cloud based backup of the whole collection