I just got my income letter at the new Australian Embassy in Bangkok on Monday. No problems, no questions asked, although I had super statements. If you have documents from your super account take them just in case. The Thai desk clerk wouldn't have a clue about Australian Superannuation law (same as most Australian's). This is the easiest way to get an Extension.
Sorry, but how will you support yourself when you are in Thailand. That's what an IO will want to know. Having an apartment lease means you are living there and not a tourist. Having, say a 12 month lease says you aren't applying for a 2 month tourist visa but intend to stay 12 months. This may worry them.
I would say if you have a brokerage account, you can use the value of your portfolio as the 850k baht equivalent. I'm Australian, and I used my Australian Superannuation (similar I think to a US 401k) balance when I applied for my O-A in Sydney.