Hey just a thought....the dealer gave you the BS that it's been sitting "just 4 days" and the battery went dead...I'll bet it's been sitting for a lot longer because he can't unload it. I would offer him $9k and see if he wants to deal. He doesn't want you to think it's been sitting because that gives you leverage...which you have.
I wouldn't hesitate to buy a 6.0...IF the price was right. There's been enough write-ups on how to diagnose and bulletproof these, that if you factor those costs in upfront you can still come out with a good reliable truck.