To the OP, diesel or gas, Hyundai or F350, it appears you're not someone who is "into" vehicles so, the diesel part is only part of the buyer beware equation. You can just as easily spend $1000s fixing other "normal" or not so normal issues.
Example, you're looking for something older. I just bought an "older" vehicle as a beater/commuter vehicle. Very good condition, only 100k miles and I've only had "minor" issues, from my standpoint. With full maintenance, tune up and the "minor" issues, I've spent about $700 not including replacing the older (5 year old Michelins that were all cracked to ____, Michelin fans...lol) tires.
Those "minor" repairs and maint, if I took it to a shop or the dealer for all of the work would have been $3-4k, not $700. And these were truly small issues, easy to fix, no front end, brakes, drive lines etc type work.
My point is, if you're not willing and handy with a wrench, you could buy a $25k truck and easily/likely spend a couple thousand if you do it yourself. Or easily drop $10k with your favorite mechanic.
This is just my take from someone who knows vehicles, what goes wrong and how to fix them.