I had A7 for few years and C7 A6 before that. Great cars to swallow motorway miles but not super engaging. I would recommend facelift version (2015+) if this is what you consider.
I think some motorway tool + older weekend fun car make a lot of sense in the scenario you describe. Commuting those distances in something like M3 will cost you a lot in maintenance and depreciation.
Also for those boring motorway drives adaptive cruise control is brilliant - it's a shame very few used cars have it here.

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