don't worry, if you have an accident you'll pay all at once. With an online intl driver's license (scam), no insurance will cover you. Also, intl driving permit (legit one) only allows you to drive up to 90 days. Beyond 90 you have to get a Thai driver's license anyway
You need a tourist visa only if you are planning to stay in Thailand more than 60 days (60 day visa + 1 30 day extension allowed = up to 90 days).
With US passport, travelling to Thailand without any Visa = you are granted a visa exemption permit of stay of 30 days (1 30 days extension also allowed = up to 60 days).