don't bother with an agent. Process is easy and straight forward. If you use their money, your illegal from the start. Don't support further corruption.
I don't believe Australia embassy certifies income. So that leaves out the 65k method. Funds in a non-thai account won't qualify for a 1yr extension of a non-o visa. You can get a non-oa visa showing foreign funds but it also requires specific Thai health insurance and criminal background check. A lot easier to simply put the money in a Thai bank account and forget about it.
You will need to take the bike to the dlt in your provincial capital. You will need bill of sale, signed copies of your and sellers ID, residence certificate from immigration and the green book. Be sure the bill of sale is witnessed by 2 people not part of the transaction.