Don't buy new. There would be plenty of impulsive photographers like mackguyver and me who used it for five minutes then pushed it straight out onto eBay/Craigslist/Gumtree. You should get a pristine one for under $100.
If getting it used, try to get one with the latest firmware to correct for loss of AF. I've run into the problem a few times.
At BP we've got two, both the lenses have a '2' as the third digit of the serial number. Haven't had any problems with either and I often accidentally push the barrel in when putting the lens cap on. We haven't updated either.
From what I understand pushing the barrel back into the body whilst the camera is switched on and still awake can give rise to the malfunction.