Personally I would wire brush any rust on the inside then pressure wash it then immediately light a fire in the firebox to dry everything then I would re season it. I believe there are instructions on the Lang site on exactly how to season it. As for the outside not long ago I wire brushed my whole smoker and repainted it that would be up to you maybe just do the firebox and warming cabinet or maybe nothing at all till it gets worse than it is now.
As for the bolt find a friend with a set of torches or hit a local welding shop and ask them to cut it off