I think the best piece of advice I could give any one thinking of building a koi pond is join your local koi club 

The pond is informal in shape with about 4,800 gallons of water. The depth is 5 feet at the deep end.

The pond was constructed in 2003 over a period of 3 months and holds approximately 800 gallons, its lined using a box welded pond liner, which was made for my pond using my measurements.