These will run 400k+ miles if you change the chain, tensioner and rails every 100k or so and do frequent engine oil changes. Valve seals would be worth doing - far cheaper than a new engine, if your compression is good. Check the latter first. Low compression would suggest a rebuild or replacement.
One pro on this board has referred to a cuatomer's 420 engine (same design family as your 560) with over 700k on it, albeit after 2 blown chains as the 100k replacement was not done. Bottom line is that, if you care for the engine, the car will rot before the engine wears out. Moving to California helps with the latter, if doing nothing for your income taxes.
As for the creaks, many of these emanate from the doors' movement in their frames, especially if your car has the vinyl door frame trim in contrast to the later velvet style material. If its vinyl, try some vinyl/rubber preservative on the door trim where it abuts the frame when the door is closed. Leather cleaner/conditioner where the seat blosters meet the backs can also work wonders, preventing squeaks from dry , abrading leather.