For quick rust removal nothing works quite as well as a steel wire cup brush mounted up in a small angle grinder.
You can get angle grinders out of places like Harbor Freight for cheap as in about 20 bucks. They will have the cub brushes too. Here are some links to the sort of thing I'm talking about.http://www.harborfreight.com/heavy-d...der-91223.htmlhttp://www.harborfreight.com/2-1-2-h...ush-47926.html
Be sure to wear leather gloves and eye protection when using this sort of tool.
You need not do the entire smoker unless it needs it. Just taking care of the rusty spots will do and as long as it all black it should look OK. Do feather out into the undamaged area a bit however.
After the rust has been removed, wipe it down with some sort of solvent to remove any oils an other contaminants that would keep the paint from adhering. Lacquer thinner or even rubbing alcohol works OK. You can't make it too clean. Sometimes I will even use something like Simple Green followed with a water rince and that followed by a final wipe down with alcohol. The alcohol removes some oils and helps any water left behind evaporate quickly too.
Follow that with some high heat, grill & BBQ spray paint that you can find at the local hardware store or home center. It's not expensive while this kind of paint is self priming so that's all the paint you'll need.
It usually dries to the touch pretty fast but most high temp paint will only fully cure once you get the smoker up to temp.
Don't paint the inside, just the outside surfaces.
It helps if it's a nice warm and calm day. Spray painting when it's cold and or windy can be a PITA. Even too hot and sunny can be problematic as it makes it difficult to get a nice looking finish because the paint dries too fast and you want to overlap spray wet edges.