You have to beable to do most of the maintenance on your truck so you don't have to pay a shop 90-100 dollars per hour for doing the repairs.
Things you should be able to do
Change tires you should beable to dismount and mount tires on rims
Do Brake jobs
Weld up the box if need be
Fix the electrical problems
If you buy a new truck then you don't have to worry about major repairs for atleast 3 years.
The rate for a tandem axle dump truck with a 58,000lb gvw runs at 75-80 dollars per hour now from 65 dollars per hour a year ago because the price of fuel getting higher. The price of tires is getting higher and higher even recaps are getting really expensive.
If you had a decent truck that was reliable and good on fuel you could try squeeze a living out of a dump truck. You also have to be the driver all the time soon as you put a driver on the truck there goes the profit.
If you are a excavation contractor you pretty much need a tandem axle dump because you always need a wheelbarrow to cart material around. A truck is also needed to pull a trailer to carry a machine its why the excavation guys here run a 16 ton or smaller excavator because it can be moved on a trailer pulled by a dump truck no need for a lowbed. When you have a machine that can be moved easily more jobs you can do in a day.
The way to get money with a dump is get hired on by the municipality as a contractor truck. When they are doing larger projects they may hire private contractors I know the local muni here does. Its pretty easy hauling with the muni work.