The psu is enough for typical setups. Although for sli/cf high end cards, it may not be enough.
As for the fans, those 50-60cfm fans perform no better than the case's stock fans. There is really no point changing them out. For fans of the same diameter, cooling power and noise comes hand in hand. A good quality can may be very slightly quieter while moving the same amount of air, but it's noting noticable.
If you change out the stock ones, might as well get the highest power fans, then use controllers to lower their speed. That way, you have quietness for day to day operation, and if the rig ever gets hot, you can cool it down by turning a knob. Try this, 133.6 cfm. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
But the case doesnt come with the bottom and top fan, so i suppose i can just buy of them.
The scythe looks great, but it is sleeve bearing. For DBB, i would still consider to get two Antec TriCool 120MM DBB... bad choice?
It's not bad. 79cfm is decent. Although the 3 speed doesn't matter if you hook it up to a variable speed controller. Besides, having more complex mechanism on the fan may not always be a good idea. It's better to just have a simple, solid, high power fan. Fewer things to go wrong.
For a cool running case like that, I'm sure 79cfm would be enough though. The high 133.6cfm would be overkill.