I agree that the belt is not a good resolution. I have converted to the chain drive as well which cost about $120,-. Brush less motor is something to consider but not necessary right away. I thinking about to convert to brush less motor but have not done this move yet. Full speed of the motor will shorten the life of the pump. In regards from Leimbach you should run the brush less motor no more then 8000 rpm. The cost for the leimbach brush less motor with controller is about $265,-. I hope the link will work.
Bernd
http://www.doc-gear.de/artikeldetails.php?aid=2961