My DH has a 328xi (2009) and we hase a Safety 1st Guide 65 that fits very well rear facing in his car. I probably should get pics it fits so well. :) In fact it we were able to put the passenger seat all the way back with it RF for a 22 pounder. This seat actually will let you RF until 40 lbs which is nice.
The other seat we looked at was the Maxi-Cosi Priori which fits well in smaller cars as well & got good reviews for a 3 series.
FWIW....DH & I both have convertibles in our cars. We thought it was a PITA to try to use just one between both cars and make sure it was properly installed each time. I would look at getting 2 affordable seats with good reviews and have them inspected by a pro after you install them. That way you know you're good.