AndyB24, I think you can travel in SEA without knowing any local language. But if you learn several words (greetings, counting etc) you will have advantage
Thai is a tonal language. If you plan to learn Thai, you need audio course, because it is extremely difficult to learn and distinguish tones (high, mid, low, rising, falling) only with paper book. If you want to buy phrasebook select one with Thai script. You can just select phrase and show it instead to try spell it correctly
Also I can recommend to learn several patterns in Malay (Bahasa Melayu). Some people in the South of Thailand speak Malay and Indonesian language (Bahasa Indonesia) very similar to Malay.