I LOVE my 300SD/116! I HIGHLY recommend buying one if you can find a nice one. Rust is definitely a factor, but if you buy a southern or southwestern car that is rust-free, and keep it garaged and out of the winter salt, you should be OK.
There's nothing about them that's any more or less complicated than the 123/300D...Same engine, same tranny, same suspension, same crappy ACC (except on the 81 and up 300Ds), same vacuum lock problems...Just a bigger chassis. According to the MB numbers and other test results from "back in the day", the 300SD, even though it is heavier, was actually slightly faster and had marginally better mileage figures...I don't know if that was due to different gear ratios or what.
I think some of the interior components hold up BETTER, and look better, in my opinion. The door panels are the most obvious item. They are made much differently, and don't have that wrinkle-prone trim around the windows that the 300Ds always have, or the small and often-broken map pockets. There's larger wood trim on the 300SD dashboard, and the glove box door latches don't quit working like the 123 ones are known for.
Don't hate me, 123 guys!...I love them too!
I just have to sing the praises of my 116 a little bit!
http://mercedes300sd.freeservers.com/index.html
Mike