Rust and service records would be the two most important things to me. Especially in terms of the transmission - I'd be looking for several fluid changes at that mileage.
These things are pretty bulletproof. I bought my 2008 2500 in 2013 with 106k. It now has 275k. It's still our vacation family hauler, and it's not a daily driver. It sits in the garage 28 days a month, but when we use it we're either towing or taking a long road trip, or both.
Got bit by the oil pickup o-ring and had to replace the motor at 207k back in 2021. Transmission/TC are original and still going strong. Have done 5 fluid changes in the transmission, not just a pan drop, all 16 quarts. I got a Blackbear Tune early on, and they dialed out 50% torque management - I think that's contributed mightily to the 6L90's longevity.
Cooler lines tend to start to weep at that age - I've replaced both the high-pressure hydroboost lines and the transmission cooler lines. Easy when the engine's out.
Yes, mileage sucks, when not towing I'm happy if I get 14-15 on the highway, but it has a 39-gallon tank so I can still drive 500 miles or more without stopping.
But find me another vehicle that:
1) Can seat 7 adults
2) Can carry a stack of 4x8 sheets, in the interior, tailgate closed and dry, without removing seats
3) Can handle a payload over 2000 lbs
4) Can tow 9000 lbs
5) Has 4wd and good ground clearance
6) Has a two-speed transfer case (I've pulled class 8 trucks out of ditches)
7) Can handle luggage/gear for a family of 5 for a 3-week vacation without using the roof or a carrier.
Pic for attention. I'll be buried in this thing. On several occasions, I've almost bought another one, to keep as a spare.
View attachment 489029