i don't think i'd use an old oil drum as you really don't know what was in it
Umm... oil?
Having a good fire in it before scrubbing it clean with lots of soap and water should be good enough, I reckon.
The cold-smoking option has always looked like the better option to me.
There seem to be different kinds of set-up to do this. As I said before, the link posted by RJ_cdn looks good, but there are one or two things I'm not entirely sure about - many of you know how limited my Polish is, especially RJ, so when he posted that link, I assumed he did it partly as some sort of challenge.
it requires a lot of baby sitting
In the summer I spend a lot of time in the garden, so this shouldn't be a problem.
My next decision will have to be deciding what kind of tree to plant - I have a corner in my garden where I could fit two or three trees that I could coppice. Beech (Fagus sylvatica) looks like a good option as it appears to be used traditionally for smoking food, it copes with being cut back very hard and looks nice in the winter.