For Malaysia bus service, I recommend you use TransNasional. This bus company is most reliable bus service in peninsular Malaysia. The far is quite cheap and the seatign is comfortable. Most important they are safe and punctual most of the tim (unless the bus broke down).
I agree with Lavafalls, make sure you stock your self with food and NEVER EVER by tickets from people walking around the bus stop. Go straight to the ticket booth.
As for the Thailand, i'm not pretty sure in fact, a bit worried since the southern province political climate is not good at the moment.