First of all, Welcome to Koi-bito!
With those dimensions your pond will 2250 US gallons. If you went 4' deep you would be at 3000 US gallons. 4' is a better depth for koi then 3' and most will say 4' is the minimum depth you should have for koi. I think personally I would go with filter rated for 5000 gallons on the pond you are building. To hide the filter you could build a house for it or you could plant a wall of shrubs and put it behind it. Really depends on your layout. If its close to a raised deck you could put it under the deck. Do you have a picture of your yard? That might help.
