There is a separate chamber a couple feet away that is burning the wood and the smoke is the only thing going through this unit, It is acting more like a chimney. Temp will be closer to 200-225 regulated in this unit, there will be control valve both on inlet and outlet to either let heat out or keep it in.
I am no paint guru but people make these out of wood and have regular old stickers on them that don't melt away so I am only assuming it will hold up. I have no proof.
I would be willing to spend 125 on a gallon on the paint but will talk to my local paint shop to see about getting a quart. I have messed around with painting several cars but I still won't claim to know anything about it, I just wing it.