Is there any kind of depression or gully on the land, that does not carry flood-size runoff currently? Because that would be the most logical place to put a pond; in a gully the dam can store much more water than a pond dug on the flat. But it takes more engineering ($$).
Personally I would lay out a series of swales starting at the top of the property, and have them all eventually overflow into the future pond area. On 20 acres you could have several small ponds uphill, plus a bigger one in the lowest spot. With some ponds uphill, you may not need additional wells in order to get sufficient water for irrigation. I think $$ spent on ponds is much more valuable than that spent on wells.
If you sign up for Geoff Lawton's free videos, you can see on the "Property Purchase" video how he points out logical locations for ponds. Some of the other videos also discuss pond locations, but I can't remember which ones.
http://geofflawton.com/
I have serious pond envy!
