You should get a checklist and go through it. If I am not mistaken, I think I saw one on the car section of EBay, but I don't spend a lot of time there. Here is one that I just found by Googling "used car checklist":

http://autos.msn.com/advice/articles/aischecklist.aspx

You will find more if you do the same search. It is not specific to diesels or the W123 300D, but is a good start.

Good luck. Let us know how it goes.

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85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60