so my question is i found a few new M5 and some used, should I even think a M5 is a good wiffy car.
The M5 is probably not the very best car as kiddy transport, but if your wife appreciates superb steering and braking, together with excellent seating she may find it a good car.
She does not have to use the "M" button and with the performance options set in default start-up settings, the car drives very well as a gentle and safe transport for your family.
It will of course consume a good deal of fuel whatever you do. Mine seems less economical driven on narrow winding country roads than when going for it on more open routes. And around town, economy can be dire, mine does about 7 miles to a UK gallon in town.
One thing to beware of, if you drive it, even once, you may find yourself sneaking it at every chance for the sheer fun of its responses. The Wifey excuse falls a bit flat in that situation.