In the end it was more dispair then patience, I thought I will never get it.
Well, haven't had much time yesterday - after setting the machine up, installing the first programs I did two renderings.
One which just uses the CPU and let it run for about 30 minutes, the air at the top of the machine just got a little bit warm.
The other one was by using Luxrender in hybrid mode (CPU + GPU), let this run for about 15 or 20 minutes - the air became a bit more warm than it was when doing the first render.
That's all, didn't hear much of the fan. There is a very very low noise which you can hear if there is no other noise in the room, yes, then you can hear that there is something (the fan) running - in total the nMP produces less noise than my iMac (2011) where I can hear the HDD working.
Hope I get enough time to get a bit more "into the deep" today.
I've got a 6core/D500/16GB RAM/512GB SSD nMP.