The engine will probably be good for a long time. Check it for blowby.
The transmission can lose 5th gear. The shaft is to short and the nut holding fifth gear on can fall off. No damage normally happens but it is annoying. A new shaft is the cure. With a lot of miles the synchronizer rings can wear out. People refer to that as feeling like it is "scratching" when shifting gears.
With that many miles you could be looking at replacing everything in the brake system. You could also be looking at a truck someone else already replaced the brakes.
Front end parts are suspect anytime a vehicle hits the 100k mark. From then on it is always something to think about. If the front end parts are not bad now they will be in time (the same as any vehicle). The steering box can go bad.
Obviously things like the starter, alternator, water pump, and other normal wear items may need work.
Look for oil leaks. When you come back tell us how many you found. Cummins like to mark their territory with black spots. That is usually minor but annoying. Does the truck smoke much when it is started cold? In cold weather smoke will happen. In warm weather very little or no smoke will happen for only a short time.
Do not let my list above scare you. I would look for the same things on a vehicle with 125k on it.
After you get the truck home fix the KDP immediately. That can be expensive to repair if it lets go.
Read the thread in the link below. It explains a lot about these trucks.
Beginners Thread - Read First!