Are extra case fans necessary ?

Solution
A very nice list of parts.

Your case comes with a 120mm fan mounted on top and one in the rear.
I would relocate both of those fans to the front as intakes and you should have a plentiful supply of fresh intake air.
Remember, that whatever comes in from the front will exit SOMEWHERE taking cpu and gpu heat with it.
Plus, if all of your intake air goes through the included front filters, your parts will stay cleaner.
The only change I might make would be to include a 120mm rear exit fan, primarily to direct the airflow. If you use too much exit capability, it will tend to draw in unfiltered air from adjacent openings.

Do not be afraid of heat. PC parts are built to tolerate heat.
The cpu will slow down or shut off to protect itself...
A very nice list of parts.

Your case comes with a 120mm fan mounted on top and one in the rear.
I would relocate both of those fans to the front as intakes and you should have a plentiful supply of fresh intake air.
Remember, that whatever comes in from the front will exit SOMEWHERE taking cpu and gpu heat with it.
Plus, if all of your intake air goes through the included front filters, your parts will stay cleaner.
The only change I might make would be to include a 120mm rear exit fan, primarily to direct the airflow. If you use too much exit capability, it will tend to draw in unfiltered air from adjacent openings.

Do not be afraid of heat. PC parts are built to tolerate heat.
The cpu will slow down or shut off to protect itself if it gets to 100c.

The graphics card fans will spin up when it gets to around 80c.
It will similarly protect itself.

If you want, buy a GTX1070 that uses a blower style cooler.
They do not depend so much on case cooling and will send the hot air directly out the back of the case.



 
Solution
Additional case fans really depend on the case and the hardware inside. In most cases the case fans that are provided are adequate for what is needed. I would wait before buying additional fans until I have the system built and see how the temps are. For your build you are using a decent case, and you should not need any additional fans from the beginning. But you may want to add some later.