you need the 800,000 baht in a Thai bank in your name two months before you *apply* for your O-visa extension .... so you should open an account and get that money in as quickly as possible. Best not to wait four weeks.
1. OA visa (from Australia) ... but the mandatory health insurance is a pain, plus you have to get a medical certificate and a Federal police clearance certificate.
2. Arrive on a 30-day visa exemption or a 60-day tourist visa and apply for a 90-day O-visa here, then extend it for 12 months at a cost of 1900 baht.
And check on the hoops you have to jump through before you are allowed to leave. I think it is OK now, but check on what visa options they will accept.