Rockers, jack points, Floors, bottoms of the front fenders, bottoms of the doors.
Check the rear trailing arms.

I just bought one about 2 weeks ago. I love it! Just the right amount of chrome. Mine has rust in all of the above places except the floors and trailing arms. If these were rusty that would have been a deal killer for me. When I am done restoring my 85 300D, then my 79 SD will be next. I will be repainting everything below the chrome line. The top half is excellent and will be left alone.
You wont be sorry if you get one. They have more get up then the 123, or the 126 cars with the 5cyl turbo. They have a tighter turning radius and seem to out corner the others as well. And as for the looks... Its all about the chrome, and the wood in the interior.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes