As a European, I know we have less help with PC Building (we don't have newegg etc). I'll try to speak simply if that helps XD
I prefer the 3570K, but the 2500K is fantastic. They are both quad core, so good performance from either. For overclocking, however, I would recommend the 2500K, as apparently SandyBridge processors are better at overclocking than IvyBridge. Either is good, and they are pretty similar in price. For gaming, the 2500K is fine. For editing/rendering, the 3570K is a bit better. I would say, though, that you should get an i7 if you are doing some heavy editing.
I just picked the i5 3570k for by build because it suports pci 3.0 It over clocks well and in a clock for clock performs test is often better than the i5 2500k but the i5 2500k will over clock higher. if you have the money i would go IvyBridge.
Depends on the price descrepency. If the 3570K is only $5 more (or the Dannish equivelant), then go for it. Any greater, then go for the 2500K. Ivy Bridges thermals get way out of control when overclocking, so the 2500K will most likely reach a higher overclock, though 3570k is still a respectable overclocker, and it's slightly higher IPC can help compensate for the lower OC. There are pretty similar, the only real advantage with Ivy Bridge is lower power consumption and the iGPU. PCIe 3 is a more or less a marketing gimick for the average consumer and gamer as there aren't any GPU's today that really benefit from the extra bandwidth.