Personally I wouldnt use a pond pump. I dont think they like being turned on and off lots. They also like to be run with full bore pipe work to match their outlet size. I used to run the pond pump during the day and turn it off at night and I had also reduced the pipe size. The pump lasted about 2 years. I installed a Titan pump using 50mm bore pipe and it runs constantly. Only gets turned off about 4 times a year for cleaning. Its been going for about 5 years now.
As for a 1000l/m pump I seriously dont think you will get anything like that flow rate through the spindle.
As Jonathan has said the stainless steel box I have made will act as a good heat sink and will hold around 10 litres of water so should do the job fine.