Hi,
I personally recommend either the 5 gal or 10 gal cylindrical "Gott style" or "Rubbermaid" water coolers. (like the one on
this page)
These provide just about an ideal grain bed depth and hold heat and drain well.
They can be easily converted by removing the tap, popping in a false bottom (such as Phils) and putting a stopper in the tap and a tube from the false bottom through the tap.
Cheers,
Brad