Look for the W124 Buying guide.

But things to look for

Rust.

Vac pumps

Head Casting if your into that sort of thing

Fluid leaks

how the trans shifts

this post has a lot more info with it. things to look at
http://www.peachparts.com/shopforum/showpost.php?p=2297320&postcount=4

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed