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With that many books it might be worth it to get a Kobo (assuming they can afford it) since it can group and organize books by author, series, and whatever category (aka collections) you choose. On a kindle that stuff is rather klunky.

If you have an 8gb Kobo you could experiment and see how many you can put on it. Epubs are zip files, which are compressed, and text, which compresses very well. Kindle books aren't and are definitely bigger and use more space.