To my knowledge the best one's as far as longevity are non-turbocharged (many pre-1980) but the downside is they are a lot slower than the turbo models.
1980-85 sd's are a good size car, turbo charged and have iron heads which are less likely to break up in the event of overheating (unlike my sdl that has an aluminum head). SDL is still very reliable though. They are all great cars in my opinion. I am fairly new to diesels and the forum, anyone with more experience feel free to correct me. I thought all 1980-85 SD's were turbo charged arent they?