70-200mm f/2.8 L is very heavy, I feel that 70-200mm f/4 L will be a good choice, a very sharp lens when wide open and not too heavy and let you enjoy photographic. You can check my posting in the forum for a feel of picture from 70-200mm f/4 L.
welcome to CS!
If you are interested in portraits and don't want to spend TOO much, you can consider buying a prime lens in the region of 50-85mm.
The thin Depth of Field will help with subject isolation, which is usually a boon for portraits.
Because your lens (17-40) is fairly wide-angle and your maximum aperture is not super-large, your depth of field shouldn't be so thin.