Apologies for resurrecting this thread but I had bookmarked it when doing my research, and went away thinking that the W123's ownership experience would be drastically different. Now that I have fully restored both chassis (W115 240D and W123 300D Auto), I can share that most of what has been shared on this thread is inaccurate. My ownership experience of both below:
Parts: The W115 and W123 part supply is remarkably similar. I'd say that 95% of all parts for the W123 are still available new from Mercedes. That number drops 90% for the W115 (I have compiled an exact list of parts that are NLA for my 240D). The rest are available from OEM suppliers - simply incredible for a 52/43 year old car!
Driving Experience: The W115 feels like a 4 door diesel sports car. I realize most will be skeptical when reading this. Believe me, I was so surprised when I drove mine! The handling and road feel is leagues beyond any Classic made by Mercedes in the past several decades and is vastly superior to the R107, W126, C126, W108, W116 and yes, the W123, which feels more plush, more akin to an armored barge in comparison. The W115 has hit 88 mph on the motorway, the W123 will easily hit 100 mph). Both cruise at 70mph with no issues and keep up with modern cars in city traffic very easily. Over the course of the last year, the W115 nets 36 mpg in mixed conditions, the W123 nets 32 mpg (Imperial). These figure increase to 40 mpg and 36 mpg on long motorway drives at 60 mph.
Rust: Both cars rust badly when left in the elements or driven in the winter. Surprisingly, my W115 rusts less because of the paint color (signal red), which had high levels of lead metal in the paint, which helped prevent corrosion. It's imperative to protect these cars with the proper chemical treatments and if done right, they will last longer than all of us.
Mechanical: The W115 gets a bad rep for the location of its oil filter but that messy job aside, it's W123-like in that it is a really easy car to maintain. Both diesel motors are nearly identical and the earlier series glow plugs can easily be upgraded to parallel pencil plugs for easier start ups. Both cars are designed to rely on rubber suspension parts, so if you replace anything, make sure parts are from Mercedes. They're not expensive at all, especially by modern car standards. Emphasize preventative maintenance and you, like me, will never experience a breakdown.
Looks: Arguably the most subjective. The W115 gets the most attention, especially from the fairer sex. The W123 isn't far behind but I've noticed that men pay much more attention to it. What always amazes me is the diversity of ethnicities that admire these cars. I've been stopped by British, German, African, South Asian, Australian and Scandinavian admirers in London, most of whom offer to purchase the vehicles from me.
Martin Buckley, automotive journalist for Classic and Sports Car, Classic Mercedes, Mercedes Enthusiast etc. drove my W115 last weekend and said that he thought it was an anomaly - no vehicle so easy to own has ever been so much fun to drive and so practical to use. I have little left to add.
As always body condition is paramount.
