I have no experience with Detroit engines. I've heard pro and cons about them for years. My advice to the guy was to get a good Detroit Diesel mechanic to accomplish a complete survey of them. I did tell him I know they are easy to rebuilt if there is a problem. Any other advice would be appreciated.
My take, the normally aspirated DD's are dang near bullet proof. The forced induction engines pushed to ridiculous H.P numbers (Johnson and Towers) are short lived. EDIT NOTE: DD's do like to sip motor oil. Just their nature.