I would scrap the drum idea. We found a local place that would roll 1/4 inch (48 x 24) for $250 bucks including the steel. Save yourself the trouble of welding to those barrels.
Start by insulating the firebox. Just did one myself, it was a lot of work. 16 gauge, two inches of insulation, and 1/4 inch under it all.
After you are done with the firebox, then decide if you want to insulate the whole shebang.