So if you go 20X40 you’ll be looking at 28k-35k gallons depending on how big of a shallow end you get. You’ll want a 60k SWG which leaves Pentair and Circupool. I had 35 people in the pool for hours at a blowout party and the water got cloudy. I cranked the IC60 and pump to 100% and it was clear before I went to bed 4 hours later with the SWG/Fliter combo. The DE would do fine as well, but go as big as they have because your massive amount of gallons will need filtering.
The VS pumps are the 011028 and 011056. They are basically the same pump but the 56 has variable flow as well. You won’t ever need that but if the builder has a better price on it, Take it.
I was shopping them both recently and the 56 was about $300 cheaper on sale at that time. The builder probably won’t reflect the internet sales, especially this far in advance.