Guys, this is really great news. After Nate's OpenInkpot hint, I just read
the User Guide and I must say that OI supports nearly everything I need. You can set any font, font size, bold, line spacings, margins, justification. You can set how the rotation behaves if you click the rotate button. You can choose between 90 degrees clockwise or counterclockwise, 180 degrees or 90 - 180 - 270 - 0 by each press. You can set the indicator below the text and if it should show the position in the book, page number, time, battery status. Hell, you can even set the height, font size and the offset from text for the indicator.
OI seems even a lot faster than the original firmware. There are YT-Videos like
this one where you can see how fast the OI firmware is compared to a Sony. And in
this video you can see a bit of the UI and the speed. You OI-Developers really did a great work. Thanks for that!
I think I'm going to risk it and try the N516 with OI. The problem is here in Germany ebook readers are still expensive. For a Pocketbook 360 you have to pay 240 Euro. One of the cheapest devices you can get here is the Sony PRS-300 for roundabout 180 Euro. Right now I found a special price for the N516 which is normally sold for 190 Euro, now for only 155 Euro. That's also the reason for my questions here.
Thank you all. You were a great help.
Greetz
Scheich Xodox