I will make a few comments as follows. I think the area where the burner gun flame impinges on should be firebrick, cast refractory or refractory board. This will promote clean complete combustion more quickly and allow the pot material to survive longer. If the cold rolled material is low enough carbon to weld without problems and soft enough for the required bends, I don't see a problem with it. The mild steel exchanger will have a limited life, I seem to remember CoreTen being used in similar aplications, and as You mention the new ones use stainless. The hot rolled pickled & oild material is probably more cost effective for the manufacturer than cold rolled, and welds and holds paint better than hot rolled with mill scale still on it.