Thanks guys for your help. Based on what I've read so far I'm now considering tablet instead of E-reader because tablets have higher performance and are more versatile. The only negative thing is the LCD-screen which is not that good when reading for long times. I'm looking at the Samsung Galaxy Tab 7.7 at the moment. It's quite a jump from my original plan, but why not to get a bit better device right away
The Pocketbook 912 seems to fill my demands well too. I'll look into it too.