Its relativily hard to find a duramax with under 100.000 on it, especially the older models. I picked up my 02 for 6700 from a guy that had a title loan on it that was about to expire. Mine had 281,000 on it when I purchased it. The duramax should have no problems lasting upwards of 400,000mi. An 04 ext cab with newish injectors is a good deal for $15,200, but I would probably try to take him down some. Mine also had a gauge cluster issue in that the odo and gear selection display would sometimes work. I pull the cluster and found some broken and bent pins that go from the display to the main board of the cluster. The 04 has a different cluster with the DIC instead of the message center, so it may be slightly more indepth, but is most likely fixable.
Main things that I checked on mine, or wish I would have:
*transfer case - 18mm ratchet wrench on the check plug (higher of the 2)
-GM has a huge issue with the pump wear on these causing fluid to lost while driving and then be used dry, which is very very bad
- I was unaware of this when I purchased mine and it cost me $1,700 for a GM re manufactured mine was beyond repair.
*Check the air filter good indication of type of care it has had
*look at and smell the trans dipstick (if Allison)
*look smell the engine oil dipstick
*ask if it has had any programmer or anything run on it performance wise.
*ask about general maintenance specifically the fuel filter, oil, oil filter, tranny fluid, tranny internal filter, and spin on filter
*make sure it engages to into 4hi and 4lo if it is push button
*look at the rear cab corners for rust
hope this helps, there is a whole post somewhere with checklists.