All diesel trucks today are computerized: Cummins, Duramax and Powerstroke. Ask for documentation-what was replaced. Also ask the owner how they treated the vehicle and what did they use it for. Test driving is an absolute MUST. Check the electronics to make sure everything works.
If you want to buy a used diesel truck, start off with a older Dodge Ram 5.9 Cummins- it's less computerized and it's a good truck in the end.