If you want MPG, look for a W123 1977-83 240D diesel with the 4-speed manual. I believe they're good for at least 30 MPG. But they are slow. 68 HP in a 3000 lb sedan kind of slow.
The W123 300D 1977-81 diesels and 1982-85 turbodiesels have more power, but get more like 25 MPG. 300TD is the wagon and 300SD is a larger car with the same engine.
Both are weird cars with great engines. I had a 1985 300D for a few years but never got it roadworthy. I sold it to concentrate on a pair of Geo Metros.
The W201 190D and 190E are probably pretty efficient, but I don't know much about them.