I will be going to look at a lens a guy I know is selling for a pretty reasonable price. It is the Canon EF 135mm "L" F2, which is a moderate tele with a fairly bright aperture for such a lens. If you look at this lens on Amazon it has a 5-star average rating. All the reviews I've seen are very good. It's an "L" series so is very well built and somewhat heavy. I primarily want this for its reach and the fact that it's a very bright lens for the size. I do have a 70-200mm F4 L that is really nice, but that F2 just gives so much better background blur, especially for street portraits.

At 135mm it may tend to "compress" (flatten) the subject's facial features. But if you can acquire it for reasonable money, a good deal. Nice to have "fast" glass in the bag.

For head shots generally a lens of 85mm or less would be better. With 35mm film cameras a 90mm would be a personal choice for a lot of what I assume you mean by "street portraits." And still gives you a fair "stand off" distance without over-compression.