First a few questions:
a) Do you have other vehicles in your stable like SUV's or minivan's, etc? If the e92 m3 is your only car, then I would highly suggest you get the sedan!
b) Are you taking the baby out most of the time by yourself or with the wife? (reason I ask is my friend has a son but he almost rarely if ever goes out with just the kid by himself) You can easily move the front pass. seat up for more space but not if you have a passenger.
c) Are you young and nimble? (I'm neither so E92 was out for sure!)
After trying to convince myself being a dad of 2 toddlers that I can live with a coupe, I'm SO GLAD I got the sedan. Bending your body in odd positions to put your kid into the car gets old really fast. The sedan is the minimum I think for anyone with kids and want a somewhat liveable family car.
Granted we have plenty of other cars to drive, I bought the m3 to share the driving fun and excitement on occasional weekends and those days I have to pickup the kids from daycare. Rarely is there 4 of us in the car ~ usually me + 1 kid, though I have taken both out by myself. Bottom line is if the 4 of us are going out, we grab the keys to our SUV's or my wife's CT200. We are planning to add a minivan by end of this year too.
If the M3 was the only car I could own, then I simply would not have one ~ it would be a Cayenne GTS or X5M. Kids need space for all of life's stuff and SUV's are the way to go if it's your only ride. My wife says I pack heavy but I like to be a well prepared dad. Nothing is worse than going somewhere and realized that stroller you needed is at home or emergency diaper change