The biggest concern with a 2004 will be injectors. Since this is a low mileage truck, it may likely not have reached a point where injector replacement has been necessary. It was a factory replacement type of thing, so it's also possible a caring owner had that taken care of. Ask anyone who may know, and request the service records. This would be a GM service, and recorded by VIN#.
Do you know which model engine it has (or what the VIN # is)? The LB7 was the first generation Duramax ('01-04), and most prone to the injector issue, and generally considered the least desirable. Respectable, but they simply improved often and for the better over time. The LLY was introduced in 2004, and is generally considered more reliable from an injector standpoint. It would give a couple more mpg also. Neither engine is a slouch.
Check the 8th digit of the VIN code to determine the engine. 1 is an LB7, 2 is an LLY.
I have had a couple 6 liters, both in Sierra's and Yukon's, and loved them. I will never consider one again after buying a Duramax. You will be blown away. Towing my trailer, I would get around 8-10mpg with my 6L. Hills always slowed me down, with the upshifts, etc. They were working.
With the Duramax, I tow a 33' trailer and get around 15-16mpg...don't judge, but I comfortably drive 75-80 now instead of the 65 I used to do with my 6L vehicles. I do have a 6 speed, while the 2004 you are looking at has a 5 speed. Normal highway driving, if I can keep it around 70-75, I am typically around 20mpg.
Diesel is also cheaper where I live...last year sometimes $0.50/gallon less than gas.
Those are called "Dura-Slacks" around here
Hilarious! and unnecessary for the OPs question. Thanks for the help.