Asia is very easy to travel around except for some countries - such as Laos and Myanmar - where road travel is a real pain.
I always opt for train travel, since it is my favoured form of travel, though it is usually more expensive and less comprehensive than my less liked bus travel.
For long distances (as in journeys of more than 10 hours) you should consider a plane. These can be very cheap in South East Asia though carriers such as AirAsia, Jetstar or Tiger Airways. With the large frequency of flights, you only need to organise something after your itinerary is more certain. With the amount of time you have, you will plenty of opportunities to be flexible.
If you are going all the way to Japan, I'd recommend visiting Korea as well - it is cheaper than Japan (not by much) and is one of my favourite countries in the world.
The itinerary you have is a good one, as you'll see USA, New Zealand and Australia before heading to Asia.
Just a word on costs - You will find Thailand, Laos, Vietnam and Cambodia to the cheapest, though Thailand is the most expensive of this group. China is pricier, particularly in cities such as Beijing and Shanghai. I'd put Korea on par with China, with Japan being more expensive again - not sure how it compares to the US, but my guess it would be fairly close.
The two most expensive places you will visit are New Zealand and Australia, with Australia being the place where you money will disappear fastest. Almost every traveller I've met going to Australia has underestimated the cost of food - it is definitely more expensive than both the UK and Europe. Whatever your food budget is for Australia, double it.
If you are able to work in Australia, then do so, as the strong Australia dollar against the Asian currencies is going to go a long way.
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