Assistance in finding PSU replacement for Desktop Alienware Aurora R4

Hi all, I am currently trying to fix a desktop computer for a friend. Computer doesn't boot any longer and I have tried everything I can imagine to resolve the problem without any progress.

Computer simply does not boot. No response at all when clicking the power button. I have tried pushing the power test button on the PSU but there were no response. I tried connecting the power pins on the FRONT_PANEL and unplugged the battery for a couple of minutes. If anyone have alternative solutions, feel free to share them.

This have lead to the assumption that the PSU is malfunctioned. I don't have a spare PSU to test with, so I would like to buy a replacement PSU, but I can't figure out where to buy a PSU that fits this machine. I would therefore like to ask you guys, if you could help me out on this one.

From what I can read online I believe I am in need of a 850w PSU but I am not certain about this. The machine is the following: www.dell.com/.../diagnose