Reidh,
In my experience, the 2500 cfm would be a minimum for cooling with little or no vehicle movement at or near idle. Also from experience in a cast iron sbc 355 ci making about 425hp. A 2500 cfm fan has done well with a OEM 4 core radiator using Purple Ice and a 50/50 mix of antifreeze.
I...
The first caution that comes to mind is DC motor life. They seem to range, these days, from 2000 to 9000 hours; depending on how deep your pocket is. How much horsepower are you making? How much air are you getting to the radiator. I'd guess at a minimum you'd need at least 2500 cfm through a...