Merino wool products are great and would suit long journeys fine.
Merino wool naturally adjusts to suit climate, remains warm when wet, ultra soft, lightweight, highly breathable, easy care, easy packable, non creasing and the big bonus, naturally odour free.
I have merino wool stuff by:
Icebreaker (zip top & long johns)
Merrell (mediumweight s/s T-shirt)
Craighoppers (lightweight l/s T-shirt)
Trekmates (lightweight s/s T-shirt and liner Gloves),
SealSkinz (waterproof socks)
Go-Outdoors (explorer socks)
Icebreaker stuff is the best, but the dearest.
Not got any underwear but Icebreaker appear do quite a few different products
http://www.merinowoolshop.co.uk/mens-merino-wool-underwear.asp