I'm a military diesel mechanic and here's my advice:
Pay a qualified heavy diesel mechanic to conduct a THOROUGH inspection of the entire vehicle, not just the drive train.
I spent 4 1/2 hours performing the inspection and drive test before we purchased our bus. I knew about the age and condition of the tires, the checked rubber air line between the frame rails, the amount of blow by, the serviceability of the air conditioners as well as the heaters, defroster, etc., the condition and adjustment of the brake pads and the air system in general, and yes, a lot of time looking over the engine bay.
Our bus has the DT444E engine as well. So does our F350 Power Stroke (called the 7.3) and the engine is a heck of a work horse. The DT444E is rated for medium heavy equipment up to 75,000 GVWR so in our 40' bus at 31,800 gross it's having a nice day.
The "B10" on the engine is 200,000 miles (meaning 10% of engines will need major maintenance which includes anything that requires dropping the oil pan, pulling a head, etc. not necessarily a rebuild. The "B50" on the engine is 350,000 miles meaning 50% of engines require major maintenance before 350,000 which ALSO MEANS that HALF of all the engines go MORE than 350,000 before needing major maintenance.
That's pretty darn good considering this engine is installed in equipment up to 75,000 GROSS.
Properly maintained the DT444E is considered a "million mile engine".
Now, is the one your looking at? Pay a mechanic to check it out.
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We've done so much, for so long, with so little, we now do the impossible, overnight, with nothing. US Marines -- 6531, 3521. . . .Ret ASE brakes & elect. Ret (auto and aviation mech). Extra Class HAM, NAUI/PADI OpenWater diver