I decided to go ahead and provide some of my thoughts on older Mercedes in case anyone is interested in getting an one.
There is an old adage that there is nothing more expensive than a cheap Mercedes. This is very accurate. I'm sure it applies equally to BMW and Jaguar, too.
Mercedes people are divided up into two camps. Diesel and Gas
Most people don't go between the two camps. I was an exception. I loved my old Diesels AND my gas Mercedes.
My knowledge is limited to older cars although I know a little about the newer models.
Starting with Diesel.
The 123 Chassis ran from 1976 to 1985 and included Gas and Diesel. 123 Diesels (240D, 300D, 300D (turbo), and 300TD (wagon)) have fanatical followings. The 240D is simple, reliable, low optioned, and SLOW. I mean SLOW. They are great cars if you are not in a hurry. Many of them had manual transmissions and this makes a significant difference. The 240D can also seem to the uninitiated with crank windows, crank sunroof, and am radios.
The 300D made from 1982-1985 is a true engineering marvel. It is a 5 cylinder workhorse with a turbo. These are the cars that set mileage records in terms of longevity. I had a 82 with 375,000, an 83 with 360,000 and an 84 with 475,000 miles. I sold it to a friend who clicked over 525,000 miles before it was crushed by his neighbor's RV. The things to look for RUST and whether it starts right up in the cold. The cold start is an excellent indicator of good maintenance. If the previous owner has taken care of it and adjusted the valves and ran good diesel and changed the oil regularly, it will start up no problem. If the valves haven't been adjusted or the oil not changed regularly, it will be very hard to start and this is usually related valves OR timing chain stretch. There are lots of little details I can tell you about to look for, but I'll save that for someone who is serious about getting one of these.
The 1985 is the hardest to find and many of them have California emissions and a different transmission and torque converter.
The Holy grail of these cars is the 1985 300CD (a two door) that is in my opinion a BEAUTIFUL WORK OF ART.
The 1980 116 Diesel 300SD was the S Class Diesel that woke up the North American market to Mercedes diesels. The 116 Chassis included some amazing cars including the 450SEL with the 6.9 engine. One of the most collectible of all Mercedes.
The 116 diesel is very hard to find, but it was a spectacular car.
The S class wasn't offered as a diesel again in the US until the 126 Chassis which came out in 1986 and 1987. This was the 300SDL. It was a long wheel base version of the 126 Chassis. These cars are HIGHLY sought after but they had serious problems with the initial head. If you find one, buy it only if the PO has records showing it has the #22 head or if you can actually read the casting number.
In 1990, Mercedes brought back the S class diesel with the 1990-91 350SDL. It was still a 126 Chassis, but it had a 3.5L diesel engine. It was a great engine, but the initial rods that were installed were not strong enough. ALL of these cars will bend the rods and then the head will start to go oval and it will burn lots of oil and eventually you will need a new engine. HOWEVER, if the PO can prove that a new block was installed (many were under warranty) and it has the new stronger rods, they are wonderful cars. Occasionally a low mileage example of these will show up and the owner will claim it has no problems. DON"T BE MISLED, all of them will need new rods it is just a matter of time.
The 140 Chassis (the S Class that replaced the 126) also had a 300SD/350D that had the same problematic engine.
124 Chassis was probably one of the most successful Mercedes. The 300E (gas) was wildly popular from 1986 through 1995. More on these later. The diesels in the 124 Chassis were good cars with the absolute best being the 1990-1993 300D 2.5. They were amazing cars. I had one and sold it to a friend with over 300,000 miles. His son ended up wrecking it with over 350,000. It had quite a bit of power, lots of low end torque and got 34 MPG. For a while a several years ago when diesel was over $5 a gallon, I actually found a source for straight soybean oil in 5 gallon containers. These cars would run on 100% soy oil (in the summer). I usually mixed 1 gallon of diesel with 5 gallons of Soybean oil and it ran perfectly! PLUS, there was much less smoke. This is the one car I wish I had back.
The 1987 300TD (wagon) is the car that fans of these cars will go crazy for. Complete turds can go for serious money, and excellent examples are priced through the roof. If you ever see one of these cheap, let me know!
The 210 Chassis replaced the 124 and the 1996-1999 E300 Diesel (300D) were again fantastic cars. The last two years were turbo diesel and routinely got 37-39 mpg. The only issue with the 210 chassis for Diesels is that the shock-towers/Spring perches rusted badly. There were a few years where Mercedes had a problem with the solution they dipped the cars into for anti-corrosion. So, if you are interested in a 210 Chassis gas or diesel, the first thing to look at is RUST. If it has rust, run away. What you don't see if MUCH worse.
After the 210 Chassis, my experience starts to drop off. The easiest rule to live by is the middle range Mercedes (of the modern 2000 and later) is the best one. The cheapest models are the cheapest for a reason. The S class is a gorgeous car that is one of the best driving cars in the world. AND they are amazing cars until they run out of warranty.
This has already been too long. So, I will save the discussion about the the Gas models for later.