I think I would go straight to a 4" pump. Just my opinion. You may not need it, but why buy a small one, then need to go buy bigger (my experience).
How much the water flows in will also be a result of what soils you have. It can make a huge difference in what you need for a pump. Possibly dig a test hole till you get to water. Before buying, might rent a pump for a day to see how much the pump can keep up, or not. See if that water just keeps flowing into the test hole, or will the pump get it down and the source slows down. The bigger your perimeter gets the more it might let water flow in, thus more the challenge in keeping up with flow.
Do you have a place to pump it to?
