If you are using income made in Thailand, it will be copies of work permit and the last 3 months of paying income tax and sso for you and for the Thai employees needed to get the work permit.
Your minimum Thai wages must be 40.000 thb a month.
I have no idea where in Thailand the OP is going to live, but I know for sure that he can go after 45 days, they will inform him to make all needed copies and let him apply or inform him he has to wait a few more days and make an appointment to get thins done on the easy way.
Get the Non Immigrant O visa before you travel, get stamped in for 90 days and after 45 days visit Immigration to apply for the 1 year Extension of Stay. Less visits to Immigration and less hassle.
Single Entry Tourist Visa gives 60 days, once the extension is an extra 30 days, than conversion to Non Immigrant O gives an extra 90 days, yes easy to do if you can meet the requirements.
No insurance needed, no police checks, no funds transfers, just get 90 days on entry and before these 90 days ends, make a border run to get another 90 days.
Correct, you have to 18 Sep to collect new documents to apply for a new visa at a Thai Embassy. Check their websites for all needed documents and prepare with your new employer before you travel.
You can but not based on being over 50 years of age, you may try based on volunteer work, being married to a Thai, depending on partner with long stay etc. The O stand for Others and not only as some people like to say retirement.