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