The Epic is a CDMA-only device, and Thailand only runs GSM. Simply put, your phone will not work on cellular networks in Thailand.
But since the phone will work on Wi-Fi, bring it anyway. You won't be able to use your own phone number, but with the help of services like Skype, you might have some way to make voice calls to the USA or elsewhere. If all else fails, at least you'll have access to the web, Facebook, E-Mail, etc.