I've owned a '80 300SD since new; it now has gone 107K miles. Its repair and maintenance cost approximates its fuel cost - about $.045 / mile fuel + $.045 / mile maintenance & repair. I do the light maintenance; the car is in excellent condition and averages 24 MPG in commute traffic.
A M-B diesel is NOT an economy car; if economy is your motivation buy a Camray or Honda Accord etc.. If, however, operation philosophy is your motivation you ought to find a '82 to '85 300D or its larger brother the
300SD. Both use the same engine and transmission, the larger car is about 10% heavier. The 300D is the more fun car to drive, the SD is the more comfortable freeway cruiser. The trunks are about the same size so it's a matter of passenger comfort vs. efficient size. I've owned both sizes; each is a nice car in its own way.
These old all cast iron diesels are quite simple engines, there is no computer, nearly everything is mechanical and an owner who can tune up a gas engine can easily maintain one of these. The maintenance is: engine oil & filter change 5K miles, valve adjustment every 15K miles, filters, transmission oil & filter change every 30K miles. IMHO these engines have a useful life of 250K to 300K miles IF REGULARLY MAINTAINED. Older cars don't get a senior discount so owners begin to skimp on the regular maintenance; you need to find a car with maintenance records that prove its maintenance history. Not many will have these records so appreciate the car that does.
Before you buy any car, no matter how good it looks, you should spend $150 to have an independent M-B shop undertake a prepurchase inspection that includes an engine compression check, which is a diesel's most telling diagnostic indication of wear. Afterward you'll know that it's either a good car to be bought with confidence or a dog to be passed on with relief.
The price range is $1,000 for cars that you don't want to $4,000 for good cars whose owners' are a bit dreamy. Thing $2,500 to $3,000 for a solid value.
Come back with your questions after you've driven some cars.