First of all congrats on the baby!
Second of all - I'm running A4 B9 saloon with a baby, now 2 y old. Finding no problems doing that.
I have had the car from the moment our baby turned 1 y old. Before that I had B7 saloon, and even that was manageable, and with that I mean that with baby seat which has a base on isofix it was really easy to lift the baby seat in and out. The one downside was, if you had a tall passenger who wanted to sit in front of the baby, it was tight, my wife had a ton of space, but not her dad - he's around 190 cm - well he could drive for an hour, but it wouldn't be comfortable after that. But in B9 we have a big baby seat from 9-36 kg Recaro Young sport hero and it's no problem to lift it in or out of the car if needs be. And also no problem lifting the baby in/out of it.
Also the luggage space is also good. We had baby stroller Stokke Xplory V5 and it fits very good inside their, leaves more than half of the space empty, as You but the two halves of the stroller on top of each other and still have a space for large bag.
We have no regrets or complaints about space in B9 saloon, we had a road trip in it, and previously in B7 we had to have a lot of stuff inside the cabin, because luggage space was cramped, but in B9 everything fitted inside trunk.
So no worries. I didn't have a preference for avant or saloon when looking for used B9 fresh out of first lease, besides the eng choice and price and was mainly concerned about getting the right options.
Audi A4 B9 3.0 TDi quattro 200 kw