Congratulations!
We have a 4.5 yr old little boy and a 10 week old girl. It all depends on how often you're using the car to take the little one out. If it is only occasionally then you'll manage with a 3dr. However, if it is the main car you transport little one around in you'll be grateful of some extra space and some extra doors.
The Group 0 car seats (0-9months) are pretty heavy without a child in them, they're also pretty difficult to manoeuvre into the back of a 3 door. If you are going put a group 0 car seat in the back of a 3dr it's definitely easier with the ones which come with a base (Maxi Cosi Pebble is 'Which?' best buy if I recall correctly). You can put rear facing car seats in the front seat, but it is always recommended you put kids in the back as they're safer.
My wife has always had a 5 door (her last car was a Panda 100hp - it was tiny, but 5 doors and a perfectly rectangular boot made it easy to live with) but for a short while I inherited a 3 door company car as a stop gap and it was a nightmare getting the baby seat in and out (had to fold front seat forward, turn car seat so she was feet first, fold down the carry handle, manoeuvre the seat in, lock it onto the base, then lean in and fold the carry handle back up - not fun especially in the rain!), was very glad to see the back of it even though it was only at weekends.
Would also recommend having as big a boot as you can get, it's amazing how much junk you have to cart around for them and no matter how small your puschair folds, they all seem to be an awkward shape and fill the boot!
Like others have said, once they are in a forward facing seat and can climb in themselves it makes life much easier, 3 doors then becomes easier to live with. It can just be awkward getting car seats in and out. Saves hassle if you buy two lots of car seats, one to keep in each car.
Having said all that, a 5dr 1 series shouldn't be a problem, sounds ideal in fact, then keep your car just for you!
Good luck!
