Any chance you can get the VIN so I can pull a window sticker?

First thing to check is if its the regular CTD or the High Output CTD. If its the H.O. it should have the Aisin trans which is much much stronger and more robust.

How many KM's on the clock and have you checked the engine hours in information screen? Cummins motors are notoriously long lasting but a 2011 would have some emissions garbage on it which can be troublesome if the truck has idled a lot in it's life. I'd rather have a truck with 170,000 kms and lower hours vs. a 70,000kms truck with a boat load of idling.

Aside from that check the rockers and bed wheel wells those are the spots that rust first. Check around floor behind seat and rear headliner for signs of water leaks as the 3rd brake light seal commonly goes on these trucks.

Check make sure the HVAC works in all positions, have seen issues in blend door system. Aside from that can't think of anything else a normal test drive wouldn't show as an issue.