I am looking at a used f250 with a diesel with 100k miles. What is the expected life of this engine prior to major OH?

So much has changed since I bought my 1978 F150!

Thanks

 

250K is typical before an overhaul, but it could be twice that. It all depends on how it is treated and the type of driving you do. Major things to check are that the coolant additive was added (to prevent cavitation) and that the airbox was working (remove and check intake for dust).