That Grundfos flex pump can even run on unregulated DC from solar cells. Cool stuff.
For less money, and if you will have 120 AC even after you go solar, you might look for a 4 inch 1/2 HP 15 GPM pump or similar. I would use a large pressure tank with around 50 or more gallons of capacity to give you at least a 15 gallon draw down. You want it efficient when the water level is 20 feet down, but you still want it to work if the water is 40 ft down.
3-wire will be a little more efficient. You would need a control box, that may or may not be provided with your pump as a kit. The peak power used during starting will be significantly less than for a 2-wire pump.
Sta-rite S15P4HS05131 would be an example.
If you will have an inverter when you put in your solar system, it may well have 240 VAC available. Running the pump on 240 would be slightly more efficient because it heats the power wires just a little less. It also might widen the list of available pumps.
You would use a little less electricity if your pressure switch was set up for 30-50 PSI than 40-60 PSI.
A 1/2 HP 12 GPM pump would be not too bad, but would not be as efficient as a 1/2 HP 15 GPM pump. A 1/2 HP 25 GPM pump would probably not have enough pressure to raise the water and pressurize your pipes efficiently. You want to stay in the efficient part of the pump curve.
I am not a pro.
