I personally would stay away from the F-650 Ford if you want a decent budget medium duty dump look at a Sterling Acterra. You can get them in any single axle gvw from 18000lbs to 35,000lbs and you can get the C-7 Cat and or the ISC Cummins (8.3).
After looking over the new Internationals I'am not impressed if I was really looking for a good single axle dump brand new it would be a T-300 Kenworth they are a little pricey but you get what you pay for.
As for split load box why do you want one if your customer wants 2 different materials you make two different trips.
You may have to have a landscape dump body with some dividers built into it.