I would think with a 16 - 18 foot dump body you are going to need a pretty darn big truck, and a chipper that's bigger than 3,000 lbs to fill it. I know that my 550 with a 12' body is about 1,500 lbs overweight when loaded with chips. A friend has an international 4300 with a 14' body, and that is probably overweight when loaded. Our chippers both weigh about 6 - 7,000 lbs. By my math if you went with an 18' box you would have about 30 yards of capacity, but be into a CDL size truck. I think most guys with that size truck are running chippers weighing in closer to 10,000 lbs.
As far as body recommendations go though, Southco Industries. Be prepared to spend somewhere around 20 - 30K for that size body, but its worth the money. Another good supplier, though I am not sure if they make them that size would be arbortech.