In reply to zimpara:
You don't need anything fancy so long as you can get it on over all the extra layers.
I used to favour an alpine bod for cold weather, but when it wore out I decided that the annoyance caused by its lack of a belay loop outweighed the gains, and went with the similar but belooped dmm super couloir.
Both cheap options, but given an infinite budget I'm not sure I would choose anything significantly different.