Socks: Get light running type socks (ultramax ironman, maybe). Then get one or two pair of thicker socks to wear for sleeping and to baby your feet in case you get some blisters if hills start to rub your feet the wrong way. The light running socks go a long way. Three or four pair is all you’ll need.
Sleeping clothes: You may not think you’re a sleeping clothes person, but then again, you may surprise yourself after wearing the same thing day after hiking day. After all, slepping and being warm in bed is a big part of a thru-hiker’s day Personally, I slept in the buck with a wonderful silk liner. I bought a beautiful sleeping bag (down) that I couldn’t bear to ruin it with the oils and dirt that were on my skin. Sleeping in the mountains of the PCT under down and silk is better than most kings have ever had it. Also the silk liner is nice in case the temp is too hot for the bag. I kept my bag so clean this way that I’ve used it a bunch since the trail, and still haven’t felt the need to wash it. --And, yes, I am a fairly clean person. The silk liner can be washed whenever you hit town, so that’s how that works
As for the bag, don’t skimp–and get something with continuous baffles so you can shake the feathers the the top or bottom. That and a silk liner and you can sleep naked and perfectly comfortable–unless you have to get up and case away a bear at night.
Underwear: Bring a pair in case you need them, but you’ll know by Idylwyld if you will need them or not. I sent mine home. I think it depends partly on how slim you are, too.
One last thing, Man, I’m getting to wordy here, bring some body glide if you are afraid of chafing. Throw away the plastic deoderant tube and just keep it in a baggie. It is one hundred times better than vasaline and isn’t oil based. Skin is often the limiting factor to endurance athletes, and this stuff is great.
That’s it. I hope that helps some. Best to you.
Johnabee