All 5 cyl 300SD years are pretty solid, but as a Californian I would avoid the 1985 which came with a trap oxidizer (emission device), renowned for shedding particles and destroying the turbo. MB had a free replacement program to replace these (after they got tired of constantly replacing turbos) with a truly passive device but I would suggest checking with your dealer before buying a 1985 to see if they will still honor that.

The general rule many follow is to buy the last year made in any model line, by which time the kinks have been worked out, but this does seem to vary from model to model. Reading this forum, the W140 chassis seems to have been exceptionally problematical in its early years (1992/3) reflecting its great complexity whereas the W126 (300SD, etc) seems to have been pretty solid most years, with the caution that you should be very careful with any six cylinder diesel which is likely to crack its head (300) or destroy its con rods (350). By contrast, the 5 cyl 617 is as solid as they come. In case you breach the faith and go with a gas engine, know that early pre-1985 380 gas engines have a single row chain that breaks if not monitored.

The 1984 had improved transmission control compared to the earlier years, so maybe that's the best year to get. Check for broken driver's seat springs - the gussetting was poorly designed/welded on early W126 chassis causing the seat to lean to one side and hurt your sciatic nerve. Easily rewelded or replaced with the differently gusseted redesigned spring - ask me how I know! Overall, the 5 cyl 300SD seems to have been remarkably free from year-to-year changes during its 1981-1985 life. Maybe others could chip in here?