For a pond of this size, you might could consider it a mud pond, with little or no filtration. It would require the population be kept small, like one fish per 5000 gallons or so, and you would not be able to see the fish, as the water would not be clear. If you were to want to add filtration in the future, then you would need to have the necessary plumbing for the pond in place going to the filter pit area. It would also require, like with most mud ponds, a complete or nearly complete pump out each year and the debris removed from the bottom. If the plumbing is there, the filtration can be added piecemeal over the years adding some of the newer technology as it comes out. You would need to be sure that the fish load and feeding regimen is such that the development of measurable ammonia and nitrite does not occur.

Now if you are looking for clear sparkling swimming pool like water, the filtration will need to be there at the onset.