Nikon 135mm f2

Got an offer to pick up the Nikon 135mm f2 from a friend (that decided Canon is better than Nikon) for around $900.  I've read some reviews and most are good with the occasional mention of purple fringing?  The lens was purchased new this past April.  I'm a portrait photographer that mostly does Seniors, Maternity, and families.  I currently have the 70-200, 28-70, and 85 f1.8.  Would this lens be a good compliment to my existing lenses?  Is $900 a good price (looked around but don't see many for sale)?  One article mentioned a ring in this lens that allowed you to really throw the background out of focus???