20 yard box on a single axle is big. I wouldnt want to be carrying that around . You wont be able to put to much material in it with out being over weight not to mention the stress of loading a large heavy box on the truck. For comparison a 17-18yd with asphalt or concrete will lift my trucks front wheels right off the ground if you are not careful.
I wouldnt worry about tearing into a used unit right away. Examine the unit very well ( piston seals, chrome, rods, cable, pins and bushings, saddles, bolts etc) dont buy a piece of junk and you should be fine.