Bill, The HP of the pump will be dependent on the depth of the well. The biggest concern in pumps is "lift". You can push water a heckuva ways without having much HP.
If you want 20,000 gallons at a time, you need to know volume of water in the well plus GPM of inflow. Then you will know whether you need a holding tank. A 20,000 gallon tank would be a whopper.
I expect you could talk to any local well driller and get some technical answers.
