Fanless PSU in semi-closed space with no forced airflow

[i.e.]

Hi,

I'm building a relatively quiet PC in a P180 Mini case – relatively quiet in this case meaning one well-behaved 120mm fan forcing little airflow into the upper section of the case, more than enough for the system inside. The case might be suboptimal, but I got it for a fair price and simply replaced the unnecessary large fan with foam.

I'm considering fanless power supplies not so much because of noise, but because I believe less moving parts equal less broken moving parts (I might've had below average luck with fans to ponder such things). However, I'm a bit concerned over the lack of airflow in the bottom section – having to add a fan for the (fanless) PSU alone kind of defeats the point, although an external fan is far easier to replace.

There's little price difference between the X-400 series and quiet yet fan-equipped PSUs, and it really is irrelevant – I'm only interested in whether the temperature stays below reasonable levels in such semi-restricted spaces as is the bottom compartment of P180 Mini.

[HFat]

It's impossible to answer your question well if you don't state how much power your box is going to require.

If you want to avoid heat buildup, you want an efficient PSU. A pico (if applicable) would move most of the heat outside the case anyway.
If price is really no object, a very efficient PSU such as the Kingwin Platinum (it goes by other names as well) reviewed by SPCR would limit the heat generated. Its fan would not spin unless you put a high load on the PSU. Efficiency is not the only factor in determining temperatures during fanless operation it matters a great deal.

I'd be surprised if there was "little difference" as you say but I don't know your location so you may be right... or you might have missed some cheap alternatives.