Driver's License and purchasing a bike

As I understand things you can legally drive on your OZ and Intl license for a maximum of 90 days.  If longer than 90 days you are required to obtain Thai license.

Things may vary slightly from one office to the next but I was not required to take a written or practical test when I obtained my license because I had a valid US license and international permit.  I did have to take color blindness, reaction time and depth perception test.

In the event you must take a written test it is available in English.

In Thailand you do not need a license to buy and register a vehicle.  You will need Copies of your passport, visa and perhaps entry card.  You will also need a letter issued by immigration confirming your address. To get this immigration may require you have a long stay visa (over 90 days) and proof of address in form of a utility bill with your name and address or perhaps a lease agreement.  If you have a work permit for Thailand then address confirmation is not usually needed.