1. The answer to "how long it would take to get the green card" varies by the option; I've already listed the timeframes above.
2. Yes, the green card allows you to work at any job in America (assuming you have the relevant education/experience/licensing/etc.), except for some types of government jobs and private-sector jobs tied to government contracts.
3. If you apply for a first-time H1B visa in the April to September time frame, you generally won't be able to use it to enter the US until October 1st due to quota limitations (there are exceptions if you will work for a quota-exempt organisation such as a university). But once you are admitted to the US with it, you can work immediately, but only for the specific company that sponsored the visa.
Does marriage or the green card inhibit this in anyway?
The H1B is a "dual intent" visa, which means you are allowed to openly pursue permanent immigration (i.e. the green card) via marriage or other routes while holding the visa. But you can't actually hold an H1B and green card at the same time, nor would you want to. The green card enables you to permanently* live and work anywhere the in the US (except for those government-related jobs I mentioned above), whereas the H1B is a temporary visa which only allows you to work for a specific company for a limited time (normally 3 years, with the option to renew for another 3).
Note that as an Australian, you have other options such as the E3 visa which has certain advantages over H1B. See
http://en.wikipedia.org/wiki/E-3_visa
*for a marriage-based green card, if you've been married for less than 2 years at the time of the interview, the card will initially be a conditional one valid for only 2 years. Close to the end of the 2 years you'll have to submit another round of paperwork to show that your marriage is genuine, and you'll possibly have another interview.