Can't help with the Sunshine Coast, but I hired a car when I went to Melbourne with my Mum and the girls a few years ago. I got a Holden Cruze and had baby seats - no problems with getting what I asked for.

FWIW, it was a struggle to fit everything - hadn't thought of boot space. Mum and I each had a suitcase - me the 70 cm odd one for me and the girls and mum one much smaller, as well as the umbrella fold double stroller and hand luggage. We made it work, but if I was going to have to do anything more than drive straight from the airport to where I was staying to dump the lot I would have gone with a bigger car.

HTH.