how much should I worry about the 120mm fan on the belly of my future psu sucking hot air from the cpu into the psu?
I measure PSU exhaust temperature with an infrared thermometer in all my thermal tests. (Yes, I know it isn't especially accurate, but I attempt to be consistent for decent relative measurements.) The PSU fan is less then an inch away from the Papst fan on my XP-120
, and the Nexus case exhaust is not much farther.
With the S12-430, the exhaust temperature under 100% sustained load is typically only 7-10C above the idle temp. In the signature configuration, the highest load temp I've seen is 37C, but is more typically 30-33C (at 19-23C ambient room temp). The idle condition is much more typical with my PC usage, and that is usually only a few degrees above room temp. BTW, the exhaust temperature dropped several degrees when I switched to that Nexus exhaust fan running at 8-12V (thermally controlled by a MB fan header).
I never made this measurement with the original Sonata TP380S PSU, but It would have been substantially higher - the exhaust air from that PSU always felt warm, unlike the Seasonic's.
So to answer your question, I don't think you have to worry at all if you're using a high quality and efficiency PSU like the Seasonic, and a decent exhaust fan.