I wore the issued Aramid socks (the thick, off white ones) with silk liners for winter. I had a full set of silk liners so I wore them all... Of course I owned them prior to, I wouldn't have bought them just for that.
I wouldn't go buy anything- except MAYBE a Camleback- and even then, find the cheapest, no frills model so it will fit under your ALICE pack.
A lot of people say headlamps, but pop on your BDU cap and see if your little issued maglight will fit in there with your enormous mellon- if it does that will work just as well. Having said that, a lamp is only like $5, not exactly breaking the bank.
They will give you knives, matches, rabbits, tent makin stuff, MREs, etc... I wouldn't buy anything crazy, and even though there are lots of things you COULD take, remember you'll be humping it all out and back and REPACKING it every morning. Takes time if you've got a lot of crap.
On flight suits. I wore BDUs in the field and a FS to prison. Many people did just fine with a FS the whole time, but I'm glad I didn't for one reason. Every camp site had a SH*Thole. Not a privy or crapper or anything else, just a hole in the ground full of sh:t (one exception was filled just shy of overflowing with a 50/50 mix of water and sh:t). I liked being able to remove my top and only have to worry about keeping my balance over the hole, not wheather or not I was dragging my sleeves...
Hope this helps.