Diesels are pretty simple, they are just different than gas cars so people get intimidated.
For a 603, just make sure all your filters are clean, check the injectors, time it with the light, check the boost, clean out the overboots line, kill the emissions stuff. Make sure you are still getting WOT on the rack when you floor the pedal, turn the ALDA screw a bit. Thats really about it.
Unlike new cars where "tune up" is just a BS word, on old diesels you really do need to tune them up ever once in awhile, its all adjustments. If you ever look at the old service scheduals for these cars they mention a lot of that stuff, like checking and lubing the throttle links every couple of years.
I was laughing because the Toyota dealer was trying to sell my mom a "tune up" on her Rav4. I was like the coolant is good for 5 years, the plugs and fuel filter 100k miles, no belt. What do they do pop an air filter in and charge you $150 for it?
New cars pretty maintenace free.