Took a bit of googling but I found a place for you to start from sorry it's not much but now you know what to look for
http://www.videouniversity.com/shop/xlr-adapter-and-a-dual-channel-mixer/
http://au.pcpartpicker.com/part/seasonic-power-supply-ssr650rm
650w psu would be good but others feel free to correct me if I am wrong. I am not 100% sure so get another opinion on this first.
Because Steam bench wouldn't be correct in my opinion. This is just a debugger/FPS tool for Linux and Steam bench sounds like something that benchmarks all Steam games. This does only Linux games.