I've had my city mini double for almost two years and I still love it. Yes, it fits through doors no problem.
I also have the adaptor for the car seat. It is great, but sort of a pain because when you fold the stroller, the adaptor sticks out.
Baby Jogger just came out with a more universal car seat adaptor which is also more user friendly.
I would select a infant car seat based on ease of use rather than if it will fit into a particular stroller.
In that case, I love my Graco Snugride 35