This is a biased opinion of course, but a later model 560 SEL, is a good bet. Post '88, so you don't run into the soft cam issue (I think that's the year). Also, the timing chain last longer than the 420 SEL. My MB tech said he has never replaced one on a 560 SEL. Plus, you have that 238 hp on tap. Worse mpg of course. Don't get wrong, 420 SEL is a wonderful car, just my opinion.