Dave,
I replaced my old non-AF-S with a new AF-S a few years ago and I, for one, will tell you that this is the best camera lens for the money from any maker, bar none. My dentist (who is better heeled than I am) has one as well as the 300 AF-S f2.8 and he and I agree the image quality is virtually identicle from these two lenses. The f2.8 obviously focuses faster and is steadier on the tripod but the f4 with both the 1.4 and the 1.7 produce spectacular results. My wife uses mine with the 1.7 almost exclusively. Check out her results at the link below. As far as new vs used is concerned, I have had extremely good luck buying used lenses. If you can possibly try before you buy, I think you'd be okay. This lens is not known for quality issues but a warranty does make most of us sleep better.
www.mary-anncampbell.blogspot.com