Should I get New Vegas? Is the ultimate edition uncut in Germany?
I can't comment on it being uncut but New Vegas is an absolutely excellent game. It improves on Fallout 3 drastically and while it was quite buggy upon release I imagine almost all of that is eliminated by now, particularly on the PC. The side quests are quite satisfying and unique, the writing a large improvement from the previous game, the world and factions are interesting, and the gameplay was similarly improved (character builds also seem much more diverse which encourages multiple playthroughs). It's hard to describe a specific aspect in comparison to Fallout 3 without simply saying it has 'improved'.
Another user (apologies that I don't remember your username) mentioned on a previous page that they prefer Fallout 3 to New Vegas (likely because Fallout 3 was played first) but concede that New Vegas is the better game and I would concur with this. The largest fault with the game is that some areas appear to have been notably cut-back due to being overly ambitious (the strip and the surrounding area in particular) but even still it is an excellent game; easily worth the €4.99 for the ultimate edition or €2.50 for the standard edition.
The one comment I'll make is that the DLC can be quite decisive but with Old World Blues included in the ultimate edition (opinions on Dead Money, Honest Hearts, and Lonesome Road seem to be quite divisive whereas I've seen little criticism, in comparison, of Old World Blues), it more than justifies the €2.50 extra for it and the ability to take a chance on the rest of the DLC (personally, I hated Dead Money and disliked Lonesome Road while I loved both Honest Hearts and Old World Blues).
Any Fallout NV demo available?
Not that I'm aware of (and I am prety certain there is not one at all). It would be quite difficult to display the game's strengths in a demo. Have you played either The Elder Scrolls series or Fallout 3?