Most portrait photographers proliferate lenses like crazy, but every time I talk with other professional photographers, I ask them what lenses they take to a typical shoot.
90% of the time, they have the same lenses in the bag. Â I'm the same way. Â Personally, I'd feel perfectly comfortable taking any portrait job in the world with just a 24-70 and a 70-200 in my bag. Â Throw in an 85mm prime and we're really in business.
If you're looking at buying a portrait lens, I'd really recommend not going too cheap. Â If you can scrap and save up to buy a 24-70 or a 70-200, you'll get a lens that will last you for many years and is so incredibly versatile that it'll save you from buying other lenses.
Last, you have to choose to get the name brand (Canon or Nikon) or the Tamron or Sigma.  Personally, I like the new crop of Tamron lenses.  I'd be very happy shooting a Tamron 70-200 for $1,000 less than the Canon with only a slight optical difference.