IT'S ALL ABOUT HISTORY!
Buying a 300,000 mile truck wouldn't be that big of a concern if you hve the proper maintenance record.
Most diesel trucks on the market today with proper maintenance and care can easily go to 200K - 300K miles. I bought my 06 Dodge with 176K miles on it. It was a one owner, all service done at local dealer, truck traded back into that same dealer which is where it was originally purchased. I have put one injector, changed oils. I know trans will need rebuilt as the bands have no adjustment left but I have been told that can be done for under $2K. The thing to look for is any sign that it has been modified with aftermarket power stuff. The items are good if they are properly installed and properly used. If they beefed it up and went to truck pulls that is different.