Get the highest CFM you can for the fan size you plan to use.
One of the variable-speed PWM fan controllers would be ideal for weak electrical systems, but they're not cheap. Lincoln Mark VIII fans move insane amounts of air, but draw something like 40A on full power and spike at least 80A on startup.

Taurus fans aren't much better.
If you want a high quality
aftermarket fan, SPAL is pretty much the leading brand. Flex-A-Lite electric fans are generally just okay and parts-store Haydens are mediocre.
Another option is a thermostatic clutch fan. Very little drag under cruise conditions, lots of airflow when needed, and no burden on the electrical system.
You might check out the electric pusher fans that come on a lot of Mercedes-Benz cars. They are low profile and powerful. It might even be possible to flip the blade and wire it in reverse to be a puller.