The best $3,000 Mercedes is one that doesnít need $3,000 of work.
But seriously nowÖ
Seeing that you list diesels as a possibility, high performance does not seem to be of significance for you. In which case the 126 380ís are plenty cheap, although you need to be aware of the timing chain problems on the early 380ís, amongst other weaknesses. The 123 diesels are a sure bet. They are solid, durable, cheap, dependable, easy to work on & readily available.
Donít overlook *any* of the later 450 series cars. They are bulletproof, rivaling the diesel engines for longevity. But they are something of gas hogs.
Make sure that the climate control is working properly on any car you look at, as this can be a major expense.
But MOST important is this: A meticulously maintained car. Donít try to buy a fixer-upper (see the first sentence above).