Sawadee khrap my friend! I'm married to a thai, therefore I know Thailand pretty well.
There is an app on the appstore, which gives you the Thai-keyboard.
Dragon Dictation is what you are looking for. It's an iPhone app, but is supposed to run fine. I've also seen some news on the net, that handwriting will come as an app too.
Even if you don't have an iPad yet, look around the US appstore. There are a lot of limited free apps, which you can download already, even w/o the iPad in hand.