With that case the front ventilation isn't brilliant, so I'd use 3 extra 140mm fans. 2x in the front in the bottom and middle slots and the remaining in the top rear-most slot. Use cardboard or plastic or even black paper under the magnetic filter to cover the rest of the Open area on top.
You'll want to use balanced or higher static pressure fans for the front 2x and a higher cfm fan for the top. Unless you are planning on using a 360mm AIO for cpu cooler, don't fill the front fan slots, just use 2x fans lower down.
Or, you could shoot for a totally negative system and just fill the top 2x slots with high cfm fans, and have no input fans at all. Just remove some of the plastic snap outs from the rear, under the gpu.
The case does not have a front dust filter, so dust accumulation isn't a factor with it without fans.