You certainly need to supply a lot more information to gauge a decent answer like your age, pre-existing conditions, amount of coverage, deductible willing to pay etc etc
you should definitely go and ask the immigration office that you will be using, as some immigration offices will allow the monthly income method and some will not.
Purchase of a Condo has absolutely nothing to do with your visa, so no just buying a Condo will not guarantee you a visa of any kind, you still must meet all the requirements for the visa you are applying for or extending.
No you don’t as an Australian passport holder you will get a 60 day visa exempt stamp when you arrive in Thailand, this is free, so you don’t have to do anything.
I actually never used it in all my years here, I prefer to go in person show your face and they remember you after a while, it certainly has helped over the years, it makes them be more lenient in my opinion, but also depends on your office.
you mentioned 90 day forms, the only form for 90 days is the TM47 for a 90 day report. If you are talking about an extension it is a TM7 form, and the TM30 is for reporting your address. If you could be more specific with your posts in the future, it will avoid confusion. Also you will only got another 30 days extension on your original 60 day visa exempt stamp, not 40 days and it will cost you 1,900 baht.