here's the Riteway made of thin steel, notice the warping. if this was 1/4" it would not warp like this. pictures speak louder than words. long term a thin stove will warp. I have seen this many times before. when buying a stove check the thickness of the plate. thicker is better. it will be heavy and harder to move around but more durable.
here is some advice from ehow, it talks about wood stoves but same applies to coal stoves for plate thicknesshttp://www.ehow.com/how_110299_buy-wood-stove.html
Understand the technology in catalytic stoves ($1,000 to $2,000). A catalytic combustor cuts normal burn temperatures in half for a slow, controlled fire with the fewest emissions. Look for a castiron or plate-steel stove body 1/4 inch (6 mm) thick and a tightclosing bypass plate 5/16 inch (8 mm) thick. Also look for a design that protects the combustor from direct flame.
Consider noncatalytic (recirculating) stoves ($500 to $2,200) for their two-chamber combustion, which injects jets of preheated air into the fire to boost heat and reduce emissions. Look for a cast-iron or plate-steel body 1/4 inch (6 mm) thick. To resist warping, the fire chamber's baffle should be 5/16-inch (8 mm) plate steel with V-shaped supports. These models have no combustor to maintain, but their smaller fireboxes mean you'll have to use shorter logs and load them more frequently.