I love wool cotton blends. The wool adds elasticity while the cotton makes it a little more breathe-able. I would recommend Cascade Sierra. It's an affordable yarn that comes in a million colors and has a ton of patterns that would work for it. This particular yarn has a larger cotton content than wool content.
If you want something with a higher wool content, and thus more insulating, I would go with Rowan Wool Cotton, which is a 50-50 wool cotton blend. However that is a pricey yarn so it would be an expensive sweater.
Hope you find something you like -a wool bamboo blend might be nice. Can't wait to find out what you choose