If you do a real "due diligence" inspection before you buy the car, (engine compression, all major systems, etc) you could be really pleased at how economical these cars are. I have gone for a year with only the 3,000 mile oil service (a MUST if you are going to keep the diesel healthy). In the eight years I've owned my 240D, rarely have costs gone over $1,000 per annum.
These cars were well thought out, simple, overbuilt, and overengineered. They represent "mature" technology, and IMHO, are the last "true" MBs built. You should be able to find a mechanically sound "driver" for maybe $2k that will continue giving yeoman service for at least another 10 years. Parts are plentiful and cheap, and sites like Mercedesshop are super valuable because they contain info on any possible problem or remedy that's been chronicled in the 20+ years these models have been on the road. They are perhaps the last autos built not to be "disposable." Style (for the W123) is wearing better than most newer models! Can't recommend them highly enough. I like the 240D, but the 300D is a fine product also.
Recommend you get a copy of Frank Barrett's "Mercedes-Benz Buyers Guide," entertaining and with a wealth of info on all MB models.