As you may know, Mercedes discontinued the wagon after 1995, the last year of the w124 chassis. The wagon was re-introduced in 1998, on the w210 chassis. The 210 came out in 1996 as a sedan, and had the m104 (DOHC) engine in 1996 and 1997, but in 1998 the ML SUV came out and all w210 chassis cars got the SOHC V6, including the wagon. Still, the w140 S-Class cars kept the m104 DOHC engine until 1999, the last year of that chassis. Specualtion is that this engine was "better" and thus was kept in the big-dollar S-Class cars. Also, the G-wagen SUV (also big $$$) kept the m104.
So, no. Don't be scared by all this, though. A well maintained SOHC V6 is probably a perfectly fine motor for a long time. I have heard that RUST! was an issue on early 210 sedans, not sure about the wagons. I couldn't believe it at first. Good luck.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro