Could be the starter itself, ring gear on the flywheel or a combination of both
Watch Kent's Youtube video and he will show you what it turned out to be on his own W113 280SL. This video will also show you how to bench test a starter.
Problem & Solution
Common among these chassis:
Classic Sedans
Ponton
R107
W108 W109
W110 W111 W112 Sedan
W111 W112 Coupe
W113
W114
W115
W116
W123
W124
W126
R129
W140