Jomtien immigration can be difficult sometimes so as a precaution do 2 copies of everything also it doesn’t hurt to photocopy your information page of your passport and also your latest entry stamp as well, just in case.
Jomtien immigration will give you the extension when you have under 7 days left on your current stamp, you will require a TM7 form filled out, a passport photo, your current TM30, your passport plus the 1,900 baht for the 30 days extension.
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.