I'm not sure what was in the last UDS that I built, I burned it out real hot, laid it on its side and slowly rolled it over letting the coals burn all the paint off the outside. I washed it out with comet and a hard bristle brush, let it dry and then started my build. I've heard of guys using all kinds of drums and some guys only use new drums so I guess it's what ever you feel comfortable with.
i have used petroleum barrels the trick is rinse them and build them the trick is seasoning them after fire basket is ready just start your coals have a bunch of fruit wood add it put your rack in set at 250 add cheap cut of meat ( bbq gods) and strips of left over butcher fat let drum smoke for 12 hrs when lid and barrel has heavy cake your ready some times it takes 2 sacrificial smokes to have barrel ready good luck