You don't have to pay stamp duty on the transfer of a used car right away, they will send you a bill for this when the seller of the vehicle has sent in the papers signed by both you and the seller. It is calculated at 3cents in the dollar on the cost of the vehicle plus a charge of I think $14, used to be $13 but it has gone up.
If I remember right you are heading to Rockingham, there is a traffic office there, get in there early though or you could be in a long queue. The bigger the vehicle the dearer the rego per year. They say bigger vehicles cause more road damage. Your 3rd party personal injury is covered in the rego and is compulsory. Private insurance ISN'T compulsory in WA.
RAC are the only breakdown service in WA. If you are a member of the RAC you will get discount on your insurance. Always ask if thats the best price they can offer you for your insurance, they may knock a bit off. I have found in the past you get a better deal if you ring rather than getting a quote on the internet. You can get a covernote as soon as you have found the vehicle you wish to buy, over the phone.
You can check if there is money owing on the vehicle or if it has been written of and rebuilt at the website for Registered Encumbered Vehicles, REVS
https://bizline.docep.wa.gov.au/revs/