The beauty of the IHS is that you can use just about any yarn that you want. If you look on the IHS knitalong website there is a list of the yarns that people have used. You could also search back through the blogs for actual photo examples.
Personally I used Malabrigo chunky and I think it's devine
It's so soft and warm and the colour is great. I used 4 skeins to complete mine with size 7mm (metric - US closest equivelent 10.5) needles. I'm a loose knitter, but I wouldn't go any bigger with the needles with that wool.
Good luck with your decision - I loved doing it so much I want to do another one!