The only "legit" places are those selling sealed boxed copies. Places like Kinguin and reddit's microsoftsoftwareswap are probably selling university program keys.
Yeah, for PSUs I'd recommend the
EVGA B2 750 watt or
GQ 650 watt for slightly better pricing.
Why not just plug in both monitors with Displayport? You don't have another DP cable? I mean, the Asus Strix GTX 980 has three DP connectors. I don't see a reason why you'd have to use HDMI.
As for whether you should get a new case and mouse.. well, if you're gonna make use of it, then it's not a waste. What's exactly wrong with the old one that you can't even close it properly? Something sticking out that's preventing the case from closing?
What's your budget? $800?
Some of those parts are old and discontinued, like the processor and motherboard. It's still better than a current gen console, though. Whether you should wait for AMD and Nvidia's next generation graphics cards will depend on how long you can wait, the timeframe for their release is still not really known except "sometime later this year".