Personally, I feel that:
1) Your child will not be at a disadvantage if she went to nursery instead of pre nursery. I have friends who don't even send their kids to nursery, and only started school proper at kindergarten stage and they are not lagging behind at all. As long as you teach your child the basics at home and expose them to socialization, seriously, I feel there is no need to send them to pre nursery.
2) I think government schools are just as competent as private ones. But that's my 2 cents and i know some do not feel the same way.
3) Open houses tend to hype things up a bit. I feel that doing a visit during school hours or asking for a tour will be much more realistic and practical.
4) I heard that you can start as early as you want. For the more popular private schools, i read about parents being on the wait list as soon as they found out that they were pregnant. Don't know how true that is though.