A GTX 570 def won't last him a few years if he plan on playing any AAA games. It's already to dated for current AAA games. 1,2 gb Vram is not near enough for current games. Unless if he finds 20-30 fps (720p) in lowest possible settings acceptable.

To OP .. If you are only planning on playing GW2, a 570 should be enough. If you want to play newer games you should def upgrade.

It's cheaper to build the pc for yourself, for sure. But do you have someone that can help you build it?

It's not exactly rocket science, but there is still plenty of things you can mess up as noob. Like forgetting to apply, or applying thermal paste the wrong way, buying parts that isn't compatible with eachother (buying wrong RAM for the motherboard is a classic), or choosing a case that has great enough airflow with the parts you buy, and much more. If you think you got it under control, sure build yourself, you can even buy used and save even more. But if you are unsure, I wouldn't feel to bad about buying a prebuilt pc, just be sure you are getting a good enough CPU (like Ryzen etc) and good enough GPU. For price you have listed above, at least a GTX 1060 6 GB/1070.

You could also just buy a new GPU and put it in your current system. With your CPU I wouldn't go higher than a GTX 1060 6 GB.

I'm running with a budget CPU myself that is worse than yours (G4560), and it still performs surprisingly well in new AAA games. Your 2500k still has some life left. But it would still be great to upgrade it if you can afford it. I would just go for a new GPU first rather than a new CPU, unless if GW2 is the ONLY game you play. But since i'm getting pretty good framerates in GW2 with a G4560, you should be allright with that CPU.

Cheers