Read a little closer, and you'll notice he says "This Mercedes mechanically is identical to the 1992 Mercedes 300SD."
Anyway, you can get a good example of a W123 300D Turbo for under $3000. I picked mine up for $2900 with a rebuilt transmission and completely overhauled climate control system. Perfect interior too. Had it checked out by a tech and everything looked good. Had 169K miles at the time. Keep your eyes open and wait for the perfect one to come around.