We bought our '05 Cooper S Convertible new when our son was six months old - we still have it and it remains our only car. We found that a folded pushchair could easily be securely wedged behind the driver's seat leaving the place behind the front passenger free for a child seat. On some trips we have had to put the rear parcel shelf on its higher setting (meaning the roof won't fold down) to get all our luggage in but we have managed to travel across the UK and holiday in Europe without any particular problems (ok, careful planning was called for on occasions!). Of course, we could have bought a "people carrier" but how much fun would that have been? However, I cannot imagine getting a pram, even a small one, in it ........................... unless it's just you and one infant, in a rear facing front seat, in which case, you could try back seats down, roof down, pram in the back, roof back up and off you go and by the time you need a rear child seat the pram may be redundant.
What? You might be planning a large family? A very large family? Don't think I can help with that one!!!