You have a pretty good idea already what to look for.
Check the front end out, they are also a known weakness, just see how well it drives down the highway.
A weak spot is the injection pump, but you can get an aftermarket one pretty reasonable and that will make it bullit proof. I think its called a VP44. Other than that, they are usually a good engine.
I own a common rail Cummins, but I think the one you are looking at is a 24 valve, and its kind of in between a pure mechanical injection, and the common rail that is controlled by the electronics like mine is. The good news is you can add a tuner to it to bump the power, plus the injectors are cheaper than common rail injectors.
Just my opinion here, but once the injector pump is upgraded, that is one of the best diesel engines, most reliable, ever made for a light duty truck.
2004 3/4 ton Quad Cab, 24 valve 5.9 Cummins