In reply to Jamesrlee:
For trousers i would go with vapour rise guides they are two layer pants which have a fleece lining and cope very well in cold temps. For your sleeping mat go with a neotherm by thermarest they are incredibly light and warm plus they pack tiny. For a fleece i would go wirh a haglofs astro cheap and brilliantly versatile they are well fitted with good quality zips and fabric had mine for 3 years and still love it 20 quid well spent. Soft shell is a funny business because do you want it to be waterproof or breathable if you go for a light weight jacket then bear in mind it might not last on a expedition. Mountain equipment softshells fit well i had a mediumbbut wear a small usually but size up if you are layering underneath. I own a rab setinel and its very warm but its fleece backed so thats another thing to consider. My choice would be.
Pants- rab vapour rise guides
Sleeping mat- thermarest neotherm
Fleece- haglofs astro
Soft shell- mid weight 2/3 pockets adjustable hood and a good fit but not tight, probs mountain equipment mission fits the bill
My experience of high altitude is non existant this is all based on multi day winter climbing/wild camping trips to wales and the lakes so take my advice with a pinch of salt
Do you not need waterproof salopettes? I would consider them