Thank you all for your opinions and recommendations. 
It seems like most of you recommend me the 1080ti, that was also my choice in the end…
…but after I saw the reviews on the net (Rakuten) and that one of the last “new” GTX1080ti on the market from Rakuten is most probably a scam I would never recieve I had no other choice as to go for the 2070 Super, as there are no ‘NEW’ 1080ti anymore on the market, at least in Germany.
Also if I would buy a used 1080ti, the risk of getting a 24/7 used Bitcoin Mining card would be too high for me.
So I looked for Discounts and found & bought a new ‘GIGABYTE GeForce RTX 2070 SUPER GAMING OC 3X 8G’ on ebay for cheap.
Also I will sell the 2 Games that are included and might get another 30€ off from the game keys. So for ~490€ I think it was a good deal, and if I’m not satisfied with the VR results I can additionally overclock the card to archive a 8% performance plus at 63°C maximum temperature, the 1080ti has over 80°C so I think the 2070 might have a longer lifespan, especially if I would have bought a used 1080ti for the same price. 
Now I’m curious at the new features of the RTX and what will be different to my GTX970 which played almost anything up-to-date on ultra @1080p. 
@nukular
Thanks for the Site.
Actually I looked again and I have a 4790K (not overclocked). I think I will overclock it for VR and stick with the old CPU and see how the results are, as a new CPU is too expensive for me right now.
@neal_white_iii
Thanks I will definitely check Quake II out. 
Yes, I heard of VorpX! I heard it is possible to play the first DeusEx in VR with it. I’m very excited to try it out as DeusEx is my all time most favorite game. 
Hope my Pimax 8K will get here soon.