Mercedes diesel 300d
The Mercedes 123 chassis is considered by many to be one of the best cars ever made. It is certainly one of the best cars Mercedes ever made. The 300D turbo diesel is, in my opinion, the pinnacle of the 123 line.
At it's core is it's basic engineering and it's simplicity. These cars do not have all the whiz bang technology of the later models. Just a rock solid body and engine combination that can easily last a half million miles with basic maintenance.
The real killer on these cars is rust. Start with a vehicle that has a rust free body and under carriage.
Next, run a compression test on all five cylinders. Good compression is paramount on a diesel engine.
These cars use vacuum extensively. The vacuum lines become old and brittle with age and can cause all sorts of gremlins from door locks to engine shut off. Fortunately the vacuum system is (relatively) easy to diagnose and repair. Vacuum lines, elbows and T's are easy to replace. Even if the vacuum pump is worn out, it's not a bad repair.
The valves need to be adjusted every 15k miles. This is critical for cold weather starts and smooth running. If you can't document the last time it was done this should be performed right off. Expect to pay about 2.5 hours of labor if done at a shop, but it can be a DIY job with the right wrenches.
There is an upgrade for the glow plug relay that makes for much better starts. The light on the dash can tell you a lot about the condition of the glow plugs themselves. If bad, I like to replace all five.
The air cleaner bracket seems to fail on all these cars at some point and should be checked out. New parts are readily available and easy to install.
The engine should run smooth and (relatively) smoke free if everything is in spec. The injection pump is rock solid and rarely fails, but it is the first place most non-Mercedes mechanics point to during diagnoses. A good clean fuel system is a must. There are three filters for the fuel system. The in-line and spin on filters are under the hood and are easy to change. I'd recommend twice a year. The third filter is often overlooked. It's the strainer filter located in the fuel tank. If algae is growing in the tank it will clog this filter and cause all sorts of problems. You can chemically kill the bugs, but the dead bugs will clog the strainer. The best bet is to remove the tank and have it cleaned, and then replace the strainer. Then buy only the best fuel from a busy station where the fuel doesn't sit for long periods.
All in all, these are very good cars and can last a very very long time. They're not hard to work on and any part on the car is readily available. Get the car up to speed on all maintenance and then keep it that way and you'll have a very enjoyable Mercedes experience.