Depending on where you are in Thailand, can be much cooler than you think. Particularly in the north it is possible to night temperatures drop to near freezing. And August is the rainy season, so it will be wet.
So let's simplify things. I will definitely stay in nylon / Gore - Tex camp. Cotton will get wet and be just heavy and useless. Long pants to protect the foot, the same for you shirt - long sleeves. Although you can pack a shirt with short sleeves. Light rain gear. And maybe a sleeping bag rated to about 32 degrees. So, a list something like this:
* Layer Base: Probably my favorite thing as a regular,
Patagonia Capilene T Silkweight One ($ 39) and bottoms ($ 45). REI's Lightweight Polartec PowerDry Crew ($ 25) is also good.
* Clothing: Pants shall be ex officio the Vent'R ($ 70) are tough and lightweight, and made for difficult trekking conditions. Columbia Sportswear's Silver Ridge II Pants ($ 72) to give you the option of convertible so you can turn them in brief.
* Clothing Rainwear: REI's Kimtah jacket ($ 229). This is a nice, light piece is made with fabric that event, which is really breathable stuff. Arc'Teryx Beta SL's ($ 250) is another good one. Made with Gore-Tex PacLite for light weight and good breathability.
* Boots: Asolo FSN Something like ($ 195), which have a Gore - Tex, leather and uppers cut nylon, and grippy outsoles for muddy trails. Lowa Khumbu Mid Boots TC of GTS ($ 245) offer similar benefits.
* Sleeping bag: Maybe Ultralamina Mountain hardwear 32 ($ 190), which has filled so that the synthetic will dry faster and keep you warm when you're wet. So there you go.
Have a good trip!