Thanks.
What do you think about configuration:
CPU -> i5 4690k
GPU -> R9 270
MoBo -> MSI H97-Gaming-3
HDD -> western digital 1tb
RAM -> hyperx fury 16gb
Power Pack-> 600w
Total -> about ~€800
It’s not a budget computer, so do you recommend me i5 4460 + R7 265 -> so I will spend ~€680?
Or maybe reduce the RAM -> not 16gb but 12gb?
8gb of ram is enough for almost anything you want to do. The only time it becomes an is when building the lighting on a large map. The LandscapeDemo on marketplace currently requires 12gb of ram to build the lights, but it is a huge landscape. An easy way to get around that limitation is to use Dynamic Lighting, then you don’t need to rebuild the lights.
I also only have 8gb of ram, have no issues whatsoever. I can run UE4, the launcher, Visual Studio, 3dsMax, Outlook, Skype, and Chrome all at the same time if I needed to. Generally I don’t have them all open at once, my system loads UE4 really quickly so I don’t have any problems closing and reopening.
I wil of course be adding another 16gb very soon, but in general 8gb is enough. 
Did you buy your PC online?