14 ft is a nice length. If you need something to base your figures/estimates off of, check the thread I posted about my tailgate trailer. It is 14 ft long, but is 7.5 ft wide. My smoker is only 30" in diameter, but its a little smaller. That might help with the scale. As long as you have good air circulation around the smoker, you can reduce the heat on your tires. As long as you plan to do indirect heating through your burn box, the actual smoker shouldnt get too terribly hot. In that case, I'd think 3" would be ok. If you ever decided to do direct heating with coals in the bottom of the smoker, then I'd worry about it heating your tires.
Using a grinding wheel with a wire attachment should work fine for knocking off the rust. That is what I did with mine. They make a rusty metal primer, which works well for the trailer. I took the grinder to my trailer to knock all the rust off, then went ahead and primed it. As I add to it, I'll just grind off where I am going to weld, and reprime it when I finish that spot. I was losing the battle with rust, and couldnt afford to get it sandblasted at the end at $60 an hour.