when my wife and I found out we were having our son - I had a 99 Honda Prelude. that was a two door with little back seats - nothing like a 911 in terms of it's performance, but the 2+2 arrangement of that car as *similar* to the 911. I could not get a car seat back there and get my son into it without breaking my neck.
whatever you get - take your car seat wtih you to the dealer and test fit it and practice/mimic what it would be like to put the baby in/out. i have taken several car seats to a car dealer- i have flat out told the dealer I want to test fit all my car seats into the car - as the ability to fit htem was part of the reason we were looking at various cars.
all those cars you mentioned have relatively small back seats but will probably fit at least one or two car seats. four doors makes it a lot easier.
the 911 sounds like the most fun and most "cool" option on your list. also the least practical as getting to the rear seats would be a PITA. I see the 911 as a great car when you child is 6. they can get in/out themselves, may no longer need a car seat but are still so small that those seats "fit" them. i don't know how well they accept a car seat... i'ms ure if you google that you will find an answer.
what about the new 4 dooor M3? not sure what your price limit is. i have heard that those are selling in the 50k range due to recession issues/poor sales.
same price point can get you into a late model M5 or RS4. both wicked fast "family" cars.
if you are like me - and need to be on a much lower budget and get something that will be somewhat fun - have you considered the Mazda RX8? i test drove one and it was the ca I would have bought had I not bought the Boxster. you can't put the top down but the back seats are FAR more accessible due to the rear doors that hinge open backward. not super fast, but about as fast as a 2.5 or 2.7L 986. similar power/weight. the car is GREAT handling - not mid engined but b/c the rotary is so light and mounted so far backward - it mimics rear engined balance. also the cockpit/layout/etc is just really really fun. it is a pure sports car, but has back seats. i reallyh reall liked that car. BTW - mazda has retroactively added a 10year/100k mile FULL warranty to the engine on ALL rX8s. even if you buy a used one from a private party that is OFF factory warranty - you get a 100k miles warranty from Mazda on the engine. wouldn't it be nice of Porsche did that for us????? you won't hear any horror stories on teh RX8 forum of someone having their motor blow with 95k miles and then having to pay a dime. test drive one - i think you would reallly like it.
is theer any chance you could buy a POS sedan like a civic, used accord, used camry, something cheap but reliable,safe - and then keep the boxster as a third car? that's what I have done. my boxster is a third/fun car. we have one good car for my wife to drive with the kids that we take on our weekend trips, and I have a DD that is a fairly good sized sedan, that is cheap/paid for/runs nice/but not much to look at. it gets me to work/back and it makes me appreciate the boxster a lot more!!
good luck
and
congratulations on the baby you will soon have!