I would never buy a used vehicle from a dealer as they have no obligation to provide repair or maintenance records for their cars and trucks. You also have no way of knowing if the previous owner(s) had a tuner installed and upped the horsepower and shortened the life of the transmission.
The LBZ was one of the best Duramax engines made and its only weakness is with the fuel system and worse case the truck goes into limp mode and there is usually no damage to the engine when this happens.
With the older diesel engines the primary concern is the fuel injectors as having them replaced will cost over $4,000.
Most people pay way too much for cars and trucks and this is especially true for diesel pickups. When I was looking to buy a used diesel truck in 2011 all the ones with less than 60,000 miles on the odometer were selling for $35,000 or more. The owners still owed the bank $35,000 on their 3 year old trucks. I decided to spend an extra $7,000 and buy a brand new Duramax instead and get the full factory warranty and the newer truck and LML engine.
Pickup truck buyers have to be the closest thing to heaven for car salesmen. I was at the Chevy dealer getting some maintenance work done on my truck and saw a 2014 1/2 ton gas powered pickup with a MSRP that was $3,000 more than what I paid for my 2500HD Duramax LT trim truck. I asked a salesman about the price and with a straight face he told me it was because of the special rims and tires.
Unless I had a burning need for a 13,000 lb. 5th wheel I would get a sub 10,000 lbs. conventional trailer and tow it with a gas engine equipped truck with 4.10 gears and stay away from diesel altogether - at this time.
If you do have to have a diesel pickup then look for one for sale by a private party who has recently paid for the fuel injectors to be replaced. A friend recently bought such a truck with 128,000 miles on it and the recent injector repairs for $22,000 which was a good deal.