At some point the idea of "use more case fans, then all of them run at lower RPM" simply turns on you and instead produce more noise. For the low wattages of this build a single case fan would've been sufficient to keep PSU and CPU fan at silent levels.

I have a few more watts in my PC case, but not a single case fan, and the machine still hovers at a comfortably quiet level at 1 meter's distance. No temperature issues.