I need some opinions..
 

Hi. I am thinking about purchasing a 2004 gmc Silverado duramax diesel. Motor: LLY
It has 160,000 klms. Asking $20,900
Anything in particular I should check or ask the salesman. Thank you. Alberta, canada

the early LLYs had overheating issues that could be easily resolved so i wouldn't worry to much about that, but it did have the potential to cause blown head gaskets. this may be hard to check unless you were to have the vehicle for a while.

look for shift smoothness of the transmission, mud in hidden places under the cab and on the frame. transfer case pump rub was also an issue on the 4x4 trucks until 2007.5,
http://www.agcoauto.com/content/imag..._rub_close.jpg

there may be others

Look to see how much it smokes, a little smoke is fine. But, when that thing really smokes could mean bad injectors or a number of things.

Check the emissions package in the glovebox. EF9's are quite mod friendly.
It might have 30-40000 left on the water pump. Pita to swap.
Both the 2 and 7 injector harnesses might be getting close to replacing.
Depending on the tire size the front end may need to be replaced.(hubs/spindles) upper and lower control arms or just the joints. Tie rod ends and pitman.
Check the right rear axle seal. Grease residue on the rim.
See if the dealer will come down bout 1500-2000.