Go the Commodore! They have a huge backseat and a huge boot! I have my two kids in booster seats in mine at the moment with heaps of room to spare. I have measured and am going to be able to fit a baby carseat in there as well. If you get a V6 they are not too bad on fuel at all. For 10-12K you would be able to get a 2000 VT in good nik.
Good luck and have fun car shopping! Buy privately as well, you'll get heaps more for your money.