The International is junk I wouldn't consider the 2500 and 2600 series trucks. As for Ford look in the 86-to about 97. I would also avoid used truck tractors they are not spec'ed heavy enough for dump applications.
For a dump you should have a chassis with a heavy single frame or a double frame. The rear axles should me minimum 44,000lbs the front axle minimum 16,000lb but 18,000lb front is prefered. The rear suspension of choice here is Hendrickson spring but any walking beam suspension is good no air ride for us.
We don't use lift axles here a tandem axle will haul legally 15 tons its all you want to carry more than that your just accelerating the wear and tear on your truck. We do run 16' boxes and a wheelbase of 222-230 I think its for spreading the weight. I know in the east you guys use tandems with 13' boxes with high sides for us thats too much concentrated weight over the tandems never get enough onto the front axle.
I would also avoid the old brands that are not even built anymore because you still need parts for those trucks. I suggest stick with Ford,Mack,Western Star,Kenworth,Pete,International and last Fruitliner.
Unless you are really mechanically inclined and have a good shop to work in stay away from the older trucks that become fixer uppers you don't want to spend more time wrenching then making money.