I just got a so called low noise Sunon 120mm fan with the Asetek Waterchill water cooling kit and I must say it is pretty noisy even at 7V. But I feel it pulls a whole lot more air then my 120mm PAPST at 7V wich is more quiet though.
You might want to look up the specs on that Sunon, and see if you can find a quieter fan with comparable airflow and static pressure ratings. However this might not be possible, TANSTAAFL, you can't get high airflow without a certain amount of noise. You might also try a lower voltage on the fan to see if it will still give you adequate cooling with lower noise.
BUT if I ever go for the option of somehow enclosing the HDs for even further noise reduction the 120mm fans will be the ones making the noise. But I am unsure if a dual 80mm radiator can cope with the extra heat of a possible watercooling sollution for the enclosed HDs.
Depends on how much capacity your cooling system has - most have enough margin that one or two drives shouldn't bother it that much. Keep in mind that modern drives are really pretty low power, in the 10-20 watts each range, so they aren't going to add all that much extra heat.