As you are US citizen you don't need a visa. Just turn up and you'll get a free 30-day entry stamp. You can extend a further 30 days at immigration office
Actually he's right. It's a grey area which the traffic police capitalise on, but under the international law which governs the issue of IDPs, if a licence is printed in English there is no requirement to have an IDP. Embassies are aware of the corrupt practices of the police and will always urge people to have one, but as far as insurance being denied due to a driver not having one, that's just poppycock!
Never used a visa agent in my life. I give advice to people which negates the need to pay bribes to government officials. My advice is always sound, unlike yours!
The OX visa which Wayne has is generally accepted by Australians as the best retirement visa, as it gives five years, which can be extended another five. If you get it through the Sydney consulate there is no requirement to transfer funds to Thailand which is a huge plus. It's worthwhile investigating if you're planning to stay awhile in Thailand and be bouncing in and out frequently