2freelife;
While not an expert, I have had a 2004 and presently have a 2002 dodge cummins. The 5.9 will probably outlast the rest of the truck. My 2004 had 212000 KMs about 165000 miles and I hauled my Arctic Fox down to Mazatlan, Mexico and back and around Alberta and B.C.. No problems. While going up and down canyons and mountains the trucks engine brake worked quite well in Tow Haul mode.
Putting in a Pac brake would be a personal choice.
I don't know the condition of your truck but I would strongly recommend getting your front end checked out. The 5.9 is a heavy engine and the front end parts should be checked out .
My 2002 that I bought strictly for towing had the once over . At 200000 KMs it needed ball joints. I changed all the oils , transmission, rear end and rad fluid. I always did an oil change as soon as I got them home and took an oil sample to get tested to establish a baseline for future tests. With the amount of miles that I had on both trucks the oil samples came back with " normal wear" indicated. Now I'll get the oil tested every oil change so I know whats going on.
The older 5.9 are great engines but please be aware that if anything goes wrong, diesels are more expensive to repair than gassers. Something to consider.
One more thing, get your valve lash adjusted if it has never been done.
If you still have the trucks manual it will tell you how many miles for maintenance. But yeah, get your oil tested and they will tell you right away if your engine has normal wear. The 5.9 will run for ever with proper care.
I love my older Cummins 5.9 but diesels are not for everyone. Kind of long winded , I know.