And regardless of how much you are draining, it takes about as long as it would to fill that amount with a garden hose. If you were draining the whole pool, it would be days. But it brings up a good point about buying one anyway. You can modify it now for more output, but you’d always have the garden hose option if you ever needed to do a water exchange. It would pump through the hose about as quick as you could fill the other end with a garden hose.
Buying the pump now would come in handy one way or another down the road. Or you could buy one just to have and rent one for this bigger project.