If this is what was holding you back, head down to the dealer right now. Cars.com reports you can indeed fit a Peg Perego car seat in the rear of a 2009 BMW M3.
[Note: reminds me of a friend who bought himself an M3 right out of business school in 1995, then promptly set to working so much, the thing barely ever left the garage. A couple of years ago, when it still had less than 30,000 miles on it, he said he was thinking what an awesome car it'd make for his son's 16th birthday present.
Then last summer he traded it in on a Mercedes E63. I mean, by 2017, kids'll be riding around in hydrogen-cell jet packs, right?