Hi and welcome.
Generally, company sponsored visas are temporary normally valid for either 2 or 4 years. It is possible to buy property on such as a visa, but you would need to get permission from the Foriegn Investment Review Board. However, you would have to consider very carefully if should a purchase is wise on a temporary visa. If she were to lose her job for any reason, then you would have a very small window of a grace period before having to leave the country.
For citizenship, you need to have been in Australia for at least 4 years with at least one year as a permanent residence