As much as I love Mercedes, I'm not a huge fan of the C-Class. But if you're only planning on buying a C-Class, then I'd say go for the sedan. I think the C-Class coupe of that time looked weird. Also, Since you said these are for long distance cruises I think a larger car would be more comfortable. I'm not really sure what your budget is but definitely aim for a midsize sedan (E-Class, 5'er, A6). I reckon you'd be able to get a 03'-04' BMW E60 or Mercedes W211 for a decent price.
Actually ever since the early 90's automatics almost always equaled or gotten better MPG than manuals. But then again that's for petrol/gasoline powered cars, I'm not sure if it's the same for diesels.