Two things: first, I have spoken to a lot to a lot of different owners of many different book readers and I have learned this, there is one reader that specializes in non-enlgish books, and it sounds like to me this is what you need. The Bookeen is the reader that I am speaking of
http://bookeen.com/en/, it may not be the simplest to use (thats the Kobo), it does not have the best methods of handling books (thats the Nook Simple Touch), its not the cheapest (thats smart phones) but it does foreign langueges such as greek. I would also recommend that you visit el.openoffice.org - this is a free office suite in greek similar to Microsoft Office.