Roughly 13,000 G. You will probably want 2 pumps, 1 in a surface skimmer to keep the surface water clean and the other could be connected to the bottom drain with a leaf strainer at the pump intake (like a pool filter has) Will you be using a waterfall at all? I have seen some nice upflow waterfall filters built using rubbermaid cattle trough. I have also seen some very capable filters built using plastic 55G barrels, bulkhead fittings and pvc, coupled with a bog filter, the water is very clean and clear. Make sure everything can be easily back-flushed, it make pond cleaning easier and less of a chore.
Will you be putting in a plant ledge for water plants, i have a ledge in my pond, it makes getting in the pond easier as well b/c you have a first step. Don't make it too large, 18" wide across one end of pond or possibly a triangular area in the corner.