In my experience if you ask an Amish man a question, the most likely answer will be "it depends." So, while I'm not Amish, my answer to the OP's original question of how many miles is "it depends." It depends on condition, price, and confidence of good maintenance. I prefer to buy new because I know how a vehicle has been cared for as it gets old. My 2012 6.7 has 116,000 mi., with no out of warranty problems except a plugged EGR. It tows our 12,500 lb. fifth wheel just great. I may routinely replace the fuel pump somewhere between 150K and 200K--won't know until that time comes, but I do expect to keep trucking. A HPFP job should be around a $1000 at a reasonably priced shop--less then 2 payments on a new truck. And, if doing that I would have the S&S bypass kit installed.
A lot of the urban legend hype around the HPFP failures resulted from the cost of repair. When the pump goes it throws metal into the injectors ruining them. Repair (on line) seems to run from $9,000 to $15,000 depending on who's telling the story. Yet, I know of a small town Ford dealer that has done them for less than $5,000. The S&S kit eliminates the injector risk, but the labor to get it installed isn't free (read fuel pump is buried.) I don't know why Ford, Cummins, and Bosch don't incorporate a bypass, but I'm sure they have a reason. Everyone does know, don't they, that Cummins now uses the CP4. Cummins is pretty conservative--doubt that they would switch without some confidence in it. And, Ford now puts the 6.7 in their F650 trucks with a 10% breakdown rating of 500,000 miles and a 250,000 mile warranty--can't be all bad, but if you buy used, you take a chance.
