Joining this thread because I am also contemplating an X1 as a baby hauler. We are not the tallest people so hoping a rear facing convertable car seat like the Radion with the angle adjuster would work.
Mr T Man regarding space I think it depends a lot on what you choose, strollers can vary a lot
in size. I'll tell you buying a stroller and car seat can be as involved as car buying, so many sizes and options to choose from. We haven't gone on a trip but do carry around a stroller, bunch of stuff, and groceries in the E90 M3 trunk. And since the baby is used to that car I'm not worried about the x1 m sport suspension being an issue, ymmv