Why don't you go to your local DIY store and buy yourself domestic heating circulator pump?
I used one and it was a marvel! Totaly silent, massive power compared to the usual WC lot ad dirt cheap. The only drawback (there must be one of course) was that it puts some heat into the water but with bong cooler it is not important as much as it would be with radiator cooled assy
Check out Grundfos pumps (europe) or any other ones since they are extremely reliable (if installed properly) and I don't think you are planning on using it for more than 20 years