Hook a monitor up to it and find out what you are going to need to throw at it... If its never had anything done its going to need some love at some point, and there is probably a reason he is getting rid of it.
Without looking at it myself I can't, nor will I tell you to buy ANY truck. What I can tell you is if you buy it without having someone who really knows these trucks do a thorough diag, you will be on here complaining. That isn't telling you not to buy it though...
You need a monitor so you can check icp, ipr, ficm, oil cooler, and pretty much every other sensor........... Take a VERY close look for ANY exhaust/boost leaks. Drive it till she is warm then keep driving for another hour, do a few WOT runs to test head gaskets. Any signs of puking at the degas bottle, well she needs head gaskets (For some people a cap will work, but most of the time it is the heads unfortunately). If the oil cooler is clogged it has been starving the egr cooler of coolant... Guess what that means...
None of these issues are a reason not to buy a truck, but if she needs head gaskets and injectors.... Well, she ain't worth 12 grand. If the truck is in fact in pristine condition with very minor issues, that price isn't bad at all and I would jump on it. But, be sure to plan to jump in and fix weak links properly WHEN they show up. If you do that you can't go wrong and you will be in it for a hell of a lot less than a cummins that probably needs a trans, injectors, has 200k on the chassis, and a front end that needs replacing every 30k miles.
Just make sure you get it at a price that makes sense considering whatever work needs done. And make sure you are ready to pull the heads and stud it properly when the day comes. And if it needs it now, PLEASE make sure you know it before you fork over $12,000. :thumb:
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Ohhhhhh and one last thing, we are here to help. Everything you could ever want or need to know is probably on this site somewhere. But, you are going to need a monitor for us to help you do pretty much anything on a 6.0.