500 SEC Starter or Starter Drive

The engine on 1984 500SEC is sometimes not turning over although the starter is turning (I can hear it). The starter looks like something I can do (have
done it on other non Merc cars) but when looking up the starter on-line to get a replacement I have a choice of getting the "starter" or a "starter drive". I've never heard of a starter drive. The starter drive is very cheap ~$10, the starter is about $120 depending on the source. Can anybody tell me what the starter drive is? Could it be that I only need this and not the whole starter? Even a DYI repair on a mercedes for $10 seems to be a dream.
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