Hi,
I'm not familiar with the 189 visa - we didn't do any research we just took a job with a 457 visa on a whim and took it from there!!
We do pay for private health insurance - about 100GBP per month, but we do have access to Medicare (Aussie NHS too). We had to get the private medical cover get the visa, but we are not clear if we still need to have it now we are in the country and registered with Medicare. We kept it anyway as we have been led to believe that Medicare is much like the NHS - OK, but waiting times are long and private medical cover allows you to get treatment more quickly.
I don't know what the childcare situation is - we are child free and haven't any information on that. If your husband gets a job with a 457 visa, then you and your family are free to stay and do what you want (work, study, sunbathe etc) as long as he is employed. I'm not sure what other security PR offers.
Mortgages - there are restrictions for 457 visa holders. We are renting but we have learnt that if we did want to buy, we would be limited to buying or building a new house, rebuilding something in poor condition or buying government housing stock (defence, social housing etc). Basically, they want you to improve Aussie housing stock.
We came here with little knowledge of Australia and so a temporary visa was fine for us. It has given us time to explore the place and decide if we want to stay and make things permanent, or move on. We have decided that we will be ready to leave on the next 12 months or so, so the 457 has been fine. Equally, if we had decided to stay, it would have been a good stepping stone to PR.
Either way, my feeling is that getting a job secured is the most important thing to do. You don't want to take a chance on it. Everything else here is easy and not much different to the UK when you have money, from buying cars and furniture to supermarket shopping and life in general, but you really don't want to be spending your savings, or the proceeds from a house sale on day to day living expenses while you find work.