Is Corsair RM850 80+ Gold PSU enough for running one Antminer C1?
Yes, although I still prefer the 1000 as an investment if you're buying a new one.
Yes, the closer to about the 50% power limit you get the better the efficiency, so a more powerful power supply will actually save you money on average, if you can find the power supply for the same price. I would recommend the EVGA 1300w G2, since you can get it for about the same price from newegg after rebate (if you don't mind MIR).