Before you buy a keyboard, try once with a usb keyboard in your reach. I found it impossible to write, it is too slow.
It was like this: You write one two words, they come letter for letter. Oh I made a mistake, it takes a while to erase, you write the next words, same again. After 5 Minutes I was really nerved and had only written one sentence.
This works only if you write as slowly as a snail.
P.S. I would love to write on e-ink.