If 3 months or less the International Driving Permit you can obtain in Aus is fine.
If longer than 3 months, come with the IDP, but during those 3 months apply for a Thai Driving licence. It is easy to obtain if you have the IDP and your Aus licence. You need a letter from immigration, medical certificate from a doctor here (easy), watch a video for an hour, do a few sight/reaction tests and you walk away with a 2 year licence.
Unsure if I read your question correctly, but if you apply for a non o retirement visa inside Thailand you do not need to buy medical insurance and it is easier to apply for than the marriage visa. You then choose and pay for whatever hospital treatment you subsequently need.
not sure I agree. Patong is horrible, but there are some nicer places in Phuket. The beaches and scenery are much nicer in Phuket. Not saying I love the place, but I would choose Phuket over Pattaya.
Depends what you want to see. If nice beaches are your thing then definitely Phuket (I would avoid Patong).
If it is temples and street food etc then Bkk is a better choice. You could catch the bus to Hua Hin to make it a two centre holiday, it's about 3 hours away.