I’d suggest dropping the requirement for “ePub, Mobi and PDF”. PDF is hopeless on a small screen, and no device does both ePub and Mobi well. Stick to either ePub or Mobi. Similarly, very few devices have an SD card slot. They can all hold 1000+ books in internal storage so it’s really not necessary.
I’d suggest looking at Kindle and Kobo. Both excellent devices and both widely available.
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