You can bring all the money you want untaxable as long as you don't stay at 180 days in this tax year and then the following year you can stay as long as you want.
O and OA are different, OA can only be bought in your home country, O can be bought in Thailand at immigration or your home country or another country. OA requires background check, and mandatory insurance.
Choose your DTV scheme and nearby embassy wisely for fast approvals and ensure you submit all required documents when you initially send them, including all pages of your passport.