You should be able to make a decent gaming rig at 1,000 euros. For Ultra settings in Arkham Knight, you'll need:
OS: Win 7 SP1, Win 8.1 (64-bit Operating System Required)
Processor: Intel Core i7-3770, 3.4 GHz | AMD FX-8350, 4.0 GHz
Memory: 8 GB RAM
Graphics Card: NVIDIA GeForce GTX 980
Graphics Memory: 4 GB
DirectX®: 11
Network: Broadband Internet Connection Required
Hard Drive Space: 55 GB
You can save roughly $100 if you switch out the i7 with an i5. You can also go with a GTX 970 instead of the 980, and save $200. I'm not sure how it'll affect performance, though. The i5 is essentially a cut i7 without any virtual cores, so, if Arkham Knight requires more than four physical cores from the i5, you might notice FPS dips. As for the GPU, the 970 has 3.5 GB of useable VRAM, but I doubt that Arkham Knight will even come close to maxing out its VRAM usage (not many games do).
I figure you'd be okay with those. The other requirements are pretty straight forward on what you need.
Thanks a ton man! I'll do some research on my own as well, because I don't know what half of these things are (I have a general idea when it comes to processors, operating systems and the such but I don't know the ins and outs of them), but this is a great starting point. Again, thanks a ton, I really appreciate it!