Chances are very good that 20K gallons will be nothing for your well. An average family of four uses a heck of a lot more than that in a year. $2000 for a well pump, wow I want to be a well digger in your area, I had a pump replaced in my last home (no pool) for $700 including labor, so two water deliveries would just about cover it. My 28K gallon pool leaked this winter, I was able to save the liner and last week I filled my pool with 28K gallons of well water and my water has a good amount of minerals and metals, I added metal out enough to treat 40K gallons of water and the only down side is, is that I have already had to backwash my DE filter once (after running it for two days) and I will have to do it again soon. It took me about three days to fill it, what you don't want to do is once you start filling it, leave the hose on, keep the pump running, all the cool water surrounding the pump will keep the motor cool and running great, the worse thing for it would be to cycle on and off. Also the calcium carbonates will most likely be very high, so you will need to treat for that. My pool is crystal clear and looks great even with the 2" of snow we had this morning! My two pennies, fill with your well. Oh one last thing, my pool installer has been doing this for over thirty years and he has had no customers hurt their well or pump from filling their pool and everyone here is on a well. Good luck.