Part of the fun would be assembling the pc yourself and the sense of achievement when you boot up your first game on it and from my small amount of experience most components come with clear enough manuals and illustrations so I'd definitely think about building it yourself and hey if you get stuck you can always ask on here
Thanks for the reply! I'll consider assembling it myself. That would save me like 20 euros if i'd have the store to assemble it.
I've changed power supply to our old desktop (Compaq Presario S6300SF xD). Also had to change some sort of Network adapter when we bought it. But that's about all the changes i've got to do so far with computers.
I tried to see what I could do with your budget, but I had trouble navigating the website you linked, however the prices seem to be very similar to that of UK sites so I'm fairly sure you could do better than a HD6850.