TBH - re-reading your original post D, a late F10 520d M-Sport as you said absolutely makes sense.

They are a lovely car indeed and the M Sports have aged will imo - the Touring's more so !

You have other cars to fulfil weekend/sunny summer days duties so put the tax/fuel savings into one of them. It also won't depreciate massively and will be a relatively easy sell on in a few years time (unlike some of your other suggestions)....