Ok - I like to throw those solutions out there because they can be economical, small footprint, passive and hence dust proof, and smaller energy footprint.
.........
CPU - I recommend against T parts as there is zero benefit for an mITX build. T and non-T CPUs follow the same voltage/frequency curve up to the point where the T parts stop. So, if the application doesn't need the extra oomph, then both parts use the same power. In other words, the T part is less benefit for the same price

Note that while the
G5400 has 2 cores, it also has hyperthreading.
case: the only dustproof case is one without fans. Now, if all you have is a CPU in there, chances are you could go with a decent cooler and never run the case fans. Web browsing/streaming isn't going to use significant CPU resources, which means there won't be significant heat build up in the case. So, don't sweat the filters...pick a case for the features you like.
cpu cooler: Scythe Kotetsu/ Kotetsu Mark II is a good choice for a 65W TDP class CPU. Fan can run down to 400rpm (300rpm for Mark II).
PSU: I've got a
Seasonic Focus Plus Gold SSR-550FX. Wattage is overkill for your needs (100W), but it is a shorter ATX supply. It has a passive mode for lower power use and it never comes on for my gaming build.