I was lookng into this ages ago when I had a Escort chargecooler.

The pump I used was called a "Nile" pump, made by LVM, part number LVM107. Was cheap, only about £45. Looks like a caravan pump, with a plastic body, but slightly larger, and 12.7mm tails. It's a 12V pump, rated for continuous running, pumping either diesel or water, and flows 12 litres per minute (which is enough). Draws 2.5A max.Can be used either inline or submerged.

There are larger pumps in the range by the same manufacturer that flow up to 36 litres per minute - have a look on Google.