Thank you all, for the comments suggestions and advice.
Update: we burned the barrels in a crazy hot fire, with a leaf blower assist, and got them mostly red-hot. However, after we put the barrel stove back into the sauna we till have that weird sweet smell. I do see some small amount of rust on the metal, but am not convinced we removed all of the galvanized coating.
We almost have everything gathered to build the rocket sauna stove, I'll be adding pictures as we attempt it!
The sauna has been heated to keep the walls drying, and because we keep thinking maybe the galvanization isn't that big a deal... but then we both seem to get mild headaches after we are working in there (even with some air flow) So, the verdict is: stay away from galvanized stuff. haha.
I am so grateful to all of you who weighed in. I have some kind of hesitation to just start experimenting with RMH stuff, even though I've watched all the movies and have some great books. It just feels like its out of my lane.
I want to come to the event and get hands on learning (I have young children, so haven't made it yet.)
Once we get the hang of it I want to build a bunch of them! everyone in this town will get dragged up here to see ours!