The gems are out there. Single owner, older owner, used diesels that haven't beem mucked with, and have low miles.
A friend of mine decided after one trip that his F150 was not enough truck to tow his new light Lance trailer. Budget constrained, he was looking for used diesels. After much research, he concluded that the 7.3L was his safest bet, despite the relatively lower prices of 6.0/6.4 trucks that were newer and had fewer miles.
It took him about 8 months, including the time lost when the first one he found got away. But to help the OP, let me describe the one he missed (because a dealer 500 miles away swooped in over the weekend, paid the private party seller 20K in cash, and flipped the truck for 24K on his lot).
The truck was a 2002 F250 crew cab short bed, pristine, single owner, garage kept, bone stock, only 70K miles, which works out to about 7K miles per year.
My friend was fairly discouraged after having that truck bought out from under him, but he kept looking. Eventually, an even better deal for his family turned up... a 2002 Excursion. It had low miles also, but for a different reason. It had a lift kit that was so tall, it wasn't practical for a daily driver.
And this is where my friend was wiser than I would have been... he bought it anyway, despite the thing looking like it belonged on a monster truck TV show. He was so embarrassed by the sight of the thing in his driveway, that he had another friend store the Excursion offsite while he gathered parts to execute his plan.
With all new front and rear springs, shocks, ball joints, steering linkage, steering gear, wheels, tires, ring and pinion gears for a ratio change, etc gathered, he brought the Excursion back home, stripped the lift kit and tires, restored the truck to optimum towing ride height with new factory parts, and then sold the lift kit, wheels and tires to offset his restoration.
His net cost for the low mileage 7.3L Excursion was about $17K. New, it was $50K+. Despite the monster lift kit and 5.13 gearing, the engine was left stock, and the transmission was already upgraded. So in essence, there was a good value used diesel hidden behind the outlandish running gear modifications.
The take away is that it might be cheaper to buy new parts to restore an older model year truck with a reliable drivetrain, than it is to buy a newer model year truck with an unreliable drivetrain. Something to think about.