Reading these posts makes me wonder how on earth we managed having an Vauxhall Astra with three young kids. We even went camping a few times in it!
Typical first child worry. We have all been there .
Having 4 doors is nice but not the end of the world. Anything Yaris sized and above is more than adequate for a family of 1 child.
The only thing you really need is a pram that works with an isofix chair. I think we paid 200ish for a full system (Half of it we barely even used so if you can buy just a frame that works with the car seat can save a bit). Just take the baby seat out and bonk it straight on the frame. No messing about.