2006 Mack Granite

Hi,
Thinking about buying a used 2006 tri axle Granite but a bit worried because the guy I would be buying it from purchased it 2 yrs ago from a used dealer in TN who has horrible reviews. Apparently he gets damaged, flooded etc trucks, repairs them then sells. From his reviews it sounds like most are not pleased with the quality of his vehicles. However, the seller I'm dealing with claims his truck has been good to him other than a blown rear end which has been repaired. He put 15 tons of sand in the dump so we could test it under load. Felt like a solid truck. Allison auto that shifted well, plenty of power etc. My concerns are....

-Black smoke when taking off from a stop. Was worse when loaded and going up an incline.(normal for a truck this age?)
-service breaks worked well but the pedal did not have a spongy feel and was a little harder to press
-VIN shows the truck as being a tandem with a 335 hp engine but it is a tri and says 375 right on the engine block?
- grill and doors have quite a bit of rust but the frame and dump bed look pretty good.

Other than these things the truck was pretty nice and the price is right. I am completely new to this so any advice would be appreciated.

Thanks