I've got a 5 Gallon Reliance Water that I replaced the spout on. For about $15 in parts from Home Depot I've got a 10 foot hose food grade hose, and pvc ball valve, reducing fitting/barb. It provides great water pressure as long as the water container is elevated above the outlet.
When selecting a water container, I'd think twice about how big you go though. At 7.5 lbs per gallon, you can add a lot of weight to your rack pretty quick and it's going to slosh around making life interesting when going around bends, stopping, etc... Plus, tossing a 150 lb water jug/bag up onto the top of your D2 would just plain suck.