LFB, sky is the limit ! Of course, you can enter all those countries, but yes, you might have to sweat to get in !
I have a friend who is currently in Russia, doing the Siberian ride up to China. He's loving it !
Russia and Kazakhstan requires invitation letters from locals to allow you in. This comes from the "old times", when the sovietic government was in power. Now, any hostel/hotel will produce this letter, for about $50. In principle, you are supposed to have booked all of your days in all of the cities you'll be visiting, but in real life, you just need to book the first days. You will have to provide an itinerary, but I've heard they don't control. Write a rough itineray.
As for Mongolia, I read the visa can be delivered upon arrival, unless you come by train. No invitation letter whatsovever. The typical documents + photos. Here you 'll find the Mongolian Embassy in Australia
• Visit this : Embassy of Russia in Australia
• A link about traveling in Russia, all infos visa etc. + culture etc. Geographica.com
• There are Kazakhstan consulates in Moscow and Ulaan Bataan (Mongolia).
• There are Mongolian consulates/embassies also in Moscow and U.B. Their addresses on this link.
Have you noticed that Mongolia and Kazakhstan's frontiers do not touch ? This means you will have to purchase a double/multiple entry visa in Russia, were you to visit both countries.
I'm going to Moscow next year for a semester in a university ! But I guess you'll be long gone by then...