I'm with Jazz on this one - for most cutting you aren't pushing the spindle hard enough to generate much heat. I don't turn the pump on for short jobs either. I use a 12V caravan water pump that is supposed to feed a tap. I have run fairly narrow pipes from a shelf under the machine via three drag chains to the spindle. Far from worrying about flow rates, my pump switch is 2-way, centre-off, so I can use 12V for filling (occasionally) but for normal use I use 5V which gives more than enough flow. Don't over-think this one - there's more significant things to worry about!

On the subject of positioning - once the pipework is full, there is effectively no real head of pressure for the pump to work against. If you can come up with a way to fill the pipes (raise the bucket temporarily?) then a small pump will handle the circulation.