I have all 680’s with Cummins engines except two are 660’s still with Cummins. The new X15 engines are very good if you can go that new. They came out in 2017. Otherwise an ISX15 with a recent rebuild would be nice.
I think a dealer salesman usually knows less about a truck for sale than a truck owner. My best buys have been from owners getting out of the business. Caused by unforseen life events. Injury, DWI death in the family. NOT selling from lack of profit. If the owner wasn't making a profit, usually they were cutting maintenance. Good luck.
I absolutely disagree, some of the worst deals I have ran into had owners who were clueless about the truck. Some owners think they are mechanically inclined and fix things so badly that the truck has to get the repair repaired.