I came over a possible solution for even lower fan noise by using a 80mm to 120mm Fan adapter:
If I remove the internal fan and place this adapter on the outside I can fit a 120mm fan on the outside. I did find a 230V AC low noise fan which I can connect after the internal ON/OFF 230V switch. But I need to drill a hole in the back plate and run the fan wires thru a rubber insulator to go inside the cabinet. If I use extra shrink tubing on the wires I think it will be ok regarding insualtion for 230V AC.
I will look into it.