Thanks for the suggestions. Meanwhile I found something really interesting -- look at this. A very nice x86 tablet, natively running Ubuntu, with a possibility to buy a 76€ case with keyboard. There are two even cheaper models but they don't support the case.

Still, I couldn't find any reviews yet. Maybe someone has some experience with them? I may be willing to risk a bit, but not too much... especially because they are not cheap at all.

Maybe this tablet deserves its own thread, I'm sure geeks would be interested.