OK here is what I did;
Dyno - I been told the engine should have 75% of it's rated power to be acceptable but I think with 300K, it should be around 90%. Also the blowby is important.
ECM dump - this tell me the hours and other info, like the firmware version for the ecm. if the hours are off, ask why - mine were within 8 hours.
VIS Check - this is a physical check of the brakes, suspension and other items - worth the trouble and expense.
I also had the dealer pull up any records they had on repairs, recalls and anything else, ditto for the engine - even though I had to go to Ocala for that info.
Had the dealer look the truck over after I bought it because the all the info I got from the three checks told me most of what was wrong.
I got one quick oil anaylis but nothing was wrong so I have another one going to blackstone when I get home.
Also I didn't but the first truck I saw, I took my time and looked. The reason I bought this truck was to make sure it was accepted into the fleet and because the engie did 95% on the dyno. I had a chance to buy a 94" sleeper with the same equipment but the engine didn't pass the ECM dump.
oh one last thing, if the owner/dealer does not let you take the time to do these things, walk away, don't waste you time with them - there are hundreds of trucks out there.