Do/Will I need extra fans?

the 400r has enough fans to start. It depends what you want though, its a good case for silence so grabbing some cheap low RPM fans will help airflow and make it silent.

Personally, I would avoid the gigabyte tri-fan models. In my experience, the dump lots of heat into the case, without offering a significant improvement over stock fans.

Do I need to worry about the positive pressure, with two intake fans and one exhaust fan?

Remember you can switch the orientation of the fans.

I honestly don't see how an intake fan helps. In my experiences yes at the focal point of the fan the air is cool but as soon as it hits the core components the air gets hot and just swishes it around the case.

In my own opinion take that back fan and have it blow all the hot air out especially depending on what heatsink and fan you are going to use.

Just wait and see until you get all your stuff and monitor the temps. Give us a full configuration of your stuff and show us the end product.

More people will help you out.

do you see what you said you don't see how a intake fan helps? are you crazy...

If i was you id pick up 2 120 mm fans for the side. and make them blow air in.*

bad idea, multiple fans is always about creating balanced flow. Airflow is most effective when quickly drawn in, and exhausted.

you want intakes on the front and side, and exhausts at the top and back.