Don't think there were 300Ds in 1993 ... there is a 300SD which had the same engine as the last W126s ... the 3.5L that had mucho problems.
W140s (starting in 1992) were far more trouble prone than the W126s ... LOTS more electronic stuff that was very very expensive to repair ... do a search under W140 and see what you think.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg