Hi and welcome to MR.
Yes, the geographical restriction is a bit of a pain but there are sites that will allow your daughter to purchase books - especially with Mum living in the US and having a credit card in the US
Check out this thread
https://www.mobileread.com/forums/showthread.php?t=43794 for some suggestions.
Also keep looking in the forum as to what books you can get where.
As to which reader is the best, have a look at this:
https://wiki.mobileread.com/wiki/E-book_devices
There are quite a few different readers out there and everyone will tell you which one is best (depends of course which one they hold in their own little hands).
If you have any specific questions, just come back to MR , post the questions and you will get heaps of friendly advice.