For the record, We got a '79 300D for our 18-year old about a year ago. Since then, it's been reliable and never failed to start. If you get one, check the condition of the vacuum pump along with brakes & suspension, since vacuum = brake assist.
His friends love riding in it, and after the past year, I'd do it all over again and will be looking for either a 300D turbo or 300 SD for my own use.
'85 F-250 6.9L Diesel