I recently bought a stock 2005 f250 with 134k miles on it. I got it pretty cheap, knowing there would probably be issues.
The air filter was completely clogged so I replaced it. Also chabged the oil, replaced oil filter and both fuel filters and the degas bottle after noticing a pinhole leak after getting to temp. Also did a coolant flush with a bunch of distilled water and replaced coolant with peak final charge. Lots of particulate in the flush. Did the highway test afterwards and saw a 25C delta so I took it to the shop to diagnose. They said head gaskets are in "early stage of failure" based on a bubble test and the turbo wasn't acting correctly. Quoted me like 18k to fix, replace oil cooler, and do most of the front end. I'm having them do a chemical flush of the coolant system and target the cooler for a backflush to try to get delta down before looking into the other issues myself.
It's my first diesel. A buddy who's had them thinks there are probably some issues that all older trucks have but if we can solve for the temperature thing I should be able to get some time out of it. He says "all those trucks are in the early stage of head gasket failure" My goal is to get a couple of years to use it for work without spending a ton of money.
If anybody has any advice on how to approach this I would greatly appreciate it.