I would say the single most important part not mentioned by the others is maintaining the cooling system. The water pumps are not the longest lived items, many of the older cars with the piston compressor AC had marginal radiators unless outfitted with the tropical kit and since the cars last so darned long, parts no one else has to think about start to fail.

In short, if you want it to last forever, start treating everything essential to the engine's operation as a wear item. On my '78 245 I spent weeks hunting down the source of a impending overheat situation (had use the heater to cool it down) when all the car needed was...every wear part in the cooling system from the radiator (badly blocked) to the water pump (impeller heavily worn) to the radiator hoses (internal leakage through the cores). With 330K miles, none of this should have surprised me but any one of the failures chould have led to a very big repair bill.