To me it's all about the test drive. You'll know right away if the truck is worth getting. I won't buy anything over 75K miles though. Once it gets over that the effort of maintaining it to near original condition starts to go up too much. As far as gas vs. diesel engine life, when I was shopping I drove a 2005 F350 6.0 that was previously in a rental fleet and it was done at 100K miles vs. an F350 V10 of the same vintage with over 300,000 miles on it in much better shape. This is admittedly an extreme example comparing a really bad diesel with a really good gas engine, but I don't think engine life is really worth worrying about in either case these days.