The problems with converted road tractors.
1) Front and rear axles too light
2) Air ride suspension the worse POS for a dump truck
3) Front axle too light
4) Light single frame will bend behind the cab
5) Axle gearing too tall
6) Wrong transmissions a 10spd isn't good
7) Wheel base is usually too short
8) Road tractors usually have high mileage
The reason why dealers convert truck tractors to dumps is to get rid of the truck. The market is flooded with used truck tractors if you see a dump truck forsale and it says unused box its a sign the truck is a conversion stay away from it.
You don't need a double frame a heavy single frame with inserts is good enough for general use. Most double frames are the same thickness as a heavy single frame. Unless your doing some heavy offroading or you have a heavy concentrated constant load on the truck then you should have double frame.
For rear suspension you can go with spring or the rubber spring type suspension like Tuff Trac or Chalmers stay away from the air ride it doesn't belong on a dump truck. For drive axle it should be minimum 44,000lb you can get away with 40,000lb if you never leave pavement 99% of the new dumps now come standard with 46,000lb rears with locks.
Front axle minimum 16,000lbs for us out here thats minimum you really need a 20,000lb front axle a 12,000lb front axle is too light.
As for brand its what ever you like the basic economy truck in todays market is Sterlings L-9500 or the Freighliner FL series. International also builds a ecomomy dump with heavy spec's. Then you get into the better trucks like the Mack Granite, Kenworth T-800 and C-500, Western Star 4900 Series, Freightliner Severe service.
The new Mack Granites are nice looking trucks and they still have Mack's toughness. My other choice would be a KW T-800 they are hard to find and are a little pricey. I do like the old A Model W-900 KWs the best they are really hard to find now but they are tough as nails