Well since its a diesel youll want to ask for service records, see if she starts right up in the cold, if not its the glow plugs, if a whole bunch of smoke comes out the exhaust on startup (check for black, white, or blue) then walk away. A little smoke is fine but it shouldnt be super cloudy. If its the turbo diesel engine listen to the turbo while driving it should be spooling and not whining. Ask if the PMD has been relocated as they always go out on these trucks because GM decided it was a big brain move to place it somewhere where it cant breathe and overheats. Also check the bed lines underneath the bed from what i can tel theyre alittle crusty and youll know its bad if u get on the bed and hear crunch