The RM650 is actually in stock and it's CHEAP!
The RM650 and RM650x are pretty much the same thing, except the RM actually has some LINK monitoring (+12V load and fan speed). Only the 750W and 850W were problematic (overheating) and they actually fixed that problem.
The only hesitation I would have is how long that PSU has been sitting around. If they have an air conditioned warehouse with a good humidity level, it should be fine. If they store their PSUs where it's hot and humid, the caps may not be in good condition anymore.