Brian,
All engines are a cr*p shoot, some will work great for one person and break down constantly for another.
The thing is, hauling has a different effect on it than say using is as a taco wagon. You as a buyer have to think about it as a tool and when you buy the tool that makes you money, then you spend some money to make sure it will work (OK I think I said that right). I tell everyone these are the most important things to do, Oil Analysis, Dyno, ECM dump and a VisCheck. the last item is not as well received but it tells you a lot about the chassis and the condition is it in. The ECM dump will tell you an awful lot about if the ECM is original, how many times the key was turned, Idle time in some of them and so on. The dyno will tell you the blow by (an indication of how the rings are) and the horse power (torque too). The OA will tell you the condition of inside the engine and so on.
The real problem with the 3126 is;
Parts are getting harder to find, which means that you may be stuck with the engine at a shop trying to find parts for it.
It uses in-cast cylinders which means that the cost to do a proper rebuild may be too much, the cylinders are part of the block, not like the Cummins ISC where you can replace them.
outside of that, it carrys the cat name and quality.
Hope that helps a bit more.