In reply to Carpe Diem:
AS mentioned it is exactly like the tourist route up the Ben except for lack of air and really cold. Same ascent and same zig zags.
It is cold and because it is popular often very crowded and slow.
Merino wool good idea, layers good, down jacket but not a great big one otherwise you will cook, generally better to have thinner layers you can progressively take off but which are not bulky in rucksack.
Disposable Handwarmers, 2 for each hand, and a few spare because they don't always work. Best to wear thinish windproof fleece gloves inside mitts so when you take your hands out of mitts they are still protected.
Insulated camelback froze in the tube despite blowing back, so make sure you also have an insulated bottle.
Good torch, Energizer lithium batteries, and best to have a small spare one, you will be plodding for hours.
The trek before is fairly easy but cold at nights. Summit day is long, 1300m and 2000m down.
Waterproof, freezeproof, dust proof camera you can strap on your belt and don't have to worry about.
Its seriously dusty, gritty since its a volcano. So use zip lock poly bags for clothes and things, the grit gets every where. Put a set of clothes, T shirt, pants, socks in a ziplock bag and when changing put old ones in bag. You kitbag will get lots of grit as will rucksack.
Take a tube of Vitamin C Orange or lemon flavoured fizzy tablets, put one in your water to help kill taste of kerosene.
Nalegene wide mouth bottle for water which you use as a hotwater bottle.
Buffs, you can breathe through them when really cold which helps keep air moist.
Take a load of throat lozenges, lockets type, need to take these to avoid sore throat because of cold dry air.
Also lip sun block and put it on all the time, from start of trek, easy to get lip blisters because of the strength of the sun with thin atmosphere, and they really hurt.