That is really up to you, the two fans that are included should be "sufficient" however adding more will lower you temps possibly up to a few degrees but it will be louder in return, its a reasonably large enough case that a 760/8350 shouldnt be too much heat for two fans
your Cooler Master K281 case supports 3 fans
It comes with 1x Front: 120mm red LED fan & 1x Rear: 120mm black fan x 1
and has the option for another 120mm side fan
If you are happy with the airflow produced by the two installed fans then don't replace them , if
not you could go with higher CFM fans to push more air throughout your case.
I would add a third optional intake fan on your side panel to help provide airflow for your 760 and also this
will give you positive airflow and help reduce dust intake into your case
If you wish to replace the fans i can recommend some high airflow fans
seeing that the third fan is optional , you can choose a quiet fan if that is what you want , i would make the side fan an intake fan to draw cool air in from outside the case especially if the single front fans airflow is partially blocked by the HDD cages
all the noctua fans are sold out on that site and not a great variety of other fans,
are there any other pc parts outlets you can deal with ? or i will make the best choice i can from what is available there ?