[HarryT]

Having owned Sony readers, which do indeed have good dictionaries, I can say that the Kindle's dictionaries are significantly better and, perhaps of interest, should you want a dictionary that's not supplied with it, there's an excellent range of good commercial dictionaries available to buy in the Kindle Store.

Given that you say that "The device will be used solely to read free, public domain works" the fact that it doesn't read ePub files is totally irrelevant; it's just as easy to find public domain works in Kindle format as in ePub, and trivial to convert ePub to the Kindle's format with tools such as Calibre. Amazon does not "keep track of what you have loaded on your Kindle".

Whether or not a tablet will "hurt your eyes" is very much a personal matter. I read an awful lot on my iPad (it's a far better choice than an eInk device for PDFs and books which benefit from colour illustrations), and I'm absolutely fine with it, although I far prefer the eInk Kindle for reading fiction. I honestly think that the fundamental problem most people have with backlit tablets is that they have the screen brightness turned up far too high.

Hope that's of some help to you.