300SD were eurotaxis. They were built for a market that puts a million miles on a vehicle, but doesn't get emotional about keeping it going past its economic life.
300SDs were extremely popular in the USA. That's why there are still so many of them running around in spite of the fact that the body is expensive to maintain. It's only relatively recently that most parts became relatively affordable, via eBay. You used to have to just pay exorbitant prices at dealers. But those parts you can't get are a serious problem. The "Febi-Bilstein" ignition lock I had to buy for my 300SD feels like garbage. I have a distinct feeling it is actually neither Febi nor Bilstein, but it's what is available.