Congrats!!

We are expecting #4 and own a Prado 7 seater.
As for car seats, Infa Secure Kompressor (XT, Crown and Caprice) are the slimmest seat you can buy, base sized. For me I have 1 Infa Secure Racer booster + Kompressor Caprice (when I buy it!) in the middle row and 2 Racers in the back row

We are looking to buy a 2nd car ASAP as I can't drive the Prado - I need auto not manual! Thinking about a Pajero, another Prado or maybe Holden Zafira. Like yourself, we need second hand and have a budget no more then 7k at the most

HTH